Why is the bid-ask spread considered a cost?
Your assets are marked to market. If you buy at X, and the market is bidding at 99.9% * X then you've already lost 0.1%. This is a market value oriented way of looking at costs. You could always value your assets with mark to model, and maybe you do, but no one else will.  Just because you think the stock is worth 2*X doesn't mean the rest of the world agrees, evidenced by the bid.  You surely won't get any margin loans based upon mark to model.  Your bankers won't be convinced of the valuation of your assets based upon mark to model. By strictly a market value oriented way of valuing assets, there is a bid/ask cost. more clarification Relative to littleadv, this is actually a good exposition between the differences between cash and accrual accounting. littleadv is focusing completely on the cash cost of the asset at the time of transaction and saying that there is no bid/ask cost.  Through the lens of cash accounting, that is 100% correct. However, if one uses accrual accounting marking assets to market (as we all do with marketable assets like stocks, bonds, options, etc), there may be a bid/ask cost.  At the time of transaction, the bids used to trade (one's own) are exhausted.  According to exchange rules that are now practically uniform: the highest bid is given priority, and if two bids are bidding the exact same highest price then the oldest bid is given priority; therefore the oldest highest bid has been exhausted and removed at trade. At the time of transaction, the value of the asset cannot be one's own bid but the highest oldest bid leftover.  If that highest oldest bid is lower than the price paid (even with liquid stocks this is usually the case) then one has accrued a bid/ask cost. "This is a misconception. One of the explanations is that if you buy at the ask price and want   to sell it right away, you can only sell at the bid price. This is incorrect. There are no two separate bid and ask prices. The price you buy (your ""bid"") is the same price someone else sells (their ""sell""). The same goes when you sell - the price you sell at is the price someone else buys. There's no spread with stocks. Emphasized it on purpose, because many people (especially those who gamble on stock exchange without knowing what they're doing) don't understand how the stock market works. On the stock exchange, the transaction price is the match between the bid price and the ask price. Thus, on any given transaction, bid always equals ask. There's no spread. There is spread with commodities (if you buy it directly, especially), contracts, mutual funds and other kinds of brokered transactions that go through a third party. The difference (spread) is that third party's fee for assuming part of the risk in the transaction, and is indeed added to your cost (indirectly, in the way you described). These transactions don't go directly between a seller and a buyer. For example, there's no buyer when you redeem some of your mutual fund - the fund pays you money. So the fund assumes certain risk, which is why there's a spread in the prices to invest and to redeem. Similarly with commodities: when you buy a gold bar - you buy it from a dealer, who needs to keep a stock. Thus, the dealer will not buy from you at the same price: there's a premium on sale and a discount on buy, which is a spread, to compensate the dealer for the risk of keeping a stock." "As an aside, on most securities with a spread of the minimum tick, there would be no bid ask spread if so-called ""locked markets"", where the price of the best bid on one exchange is equal to the price of the best ask on another, were permitted. It is currently forbidden for a security to have posted orders having the same price for both bid and ask even though they're on different exchanges. Option spreads would narrow as well as a result."
One difference is the bid/ask spread will cost you more in a lower cost stock than a higher cost one. Say you have two highly liquid stocks with tiny spreads: If you wanted to buy say $2,000 of stock: Now imagine these are almost identical ETFs tracking the S&P 500 index and extrapolate this to a trade of $2,000,000 and you can see there's some cost savings in the higher priced stock. As a practical example, recently a popular S&P 500 ETF (Vanguard's VOO) did a reverse split to help investors minimize this oft-missed cost. The most likely explanation is that the calls are being bought as a part of a spread trade.  It doesn't have to be a super complex trade with a bunch of buys or sells. In fact, I bought a far out of the money option this morning in YHOO as a part of a simple vertical spread.  Like you said, it wouldn't make sense and wouldn't be worth it to buy that option by itself. The point is that the bid and ask prices dictate what you can buy and sell at (at market, at least), and the difference between the two, or spread, contributes implicitly to your gains or losses. For example, say your $1 stock actually had a bid of $0.90 and an ask of $1.10; i.e. say that $1 was the last price. You would have to buy the stock at the ask price of $1.10, but now you can only sell that stock at the bid price of $0.90. Thus, you would need to make at least that $0.20 spread before you can make a profit. Real world example.    AGNC = 21.79 time of post.  Upcoming .22 cents ex-div Mar 27th Weekly options Mar 27th - $22 strike put has a bid ask spread of .22 / .53. If you can get that put for less than .21 after trade fee's, you'll have yourself a .22 cent arbitrage.   Anything more than .21 per contract eats into your arbitrage.  At .30 cents you'll only see .13 cent arbitrage.  But still have tax liability on .22 cents.  (maybe .05 cents tax due to REIT non-exempt dividend rates)  So that .13 gain is down to a .08 cents after taxes. This is copying my own answer to another question, but this is definitely relevant for you: A bid is an offer to buy something on an order book, so for example you may post an offer to buy one share, at $5. An ask is an offer to sell something on an order book, at a set price. For example you may post an offer to sell shares at $6. A trade happens when there are bids/asks that overlap each other, or are at the same price, so there is always a spread of at least one of the smallest currency unit the exchange allows. Betting that the price of an asset will go down, traditionally by borrowing some of that asset and then selling it, hoping to buy it back at a lower price and pocket the difference (minus interest). Going long, as you may have guessed, is the opposite of going short. Instead of betting that the price will go down, you buy shares in the hope that the price will go up. So, let's say as per your example you borrow 100 shares of company 'X', expecting the price of them to go down. You take your shares to the market and sell them - you make a market sell order (a market 'ask'). This matches against a bid and you receive a price of $5 per share. Now, let's pretend that you change your mind and you think the price is going to go up, you instantly regret your decision. In order to pay back the shares, you now need to buy back your shares as $6 - which is the price off the ask offers on the order book. Similarly, the same is true in the reverse if you are going long. Because of this spread, you have lost money. You sold at a low price and bought at a high price, meaning it costs you more money to repay your borrowed shares. So, when you are shorting you need the spread to be as tight as possible. "Bull means the investor is betting on a rising market. Puts are a type of stock option where the seller of a put option promises to buy 100 shares of stock from the buyer of the put option at a pre-agreed price called the strike price on any day before expiration day.  The buyer of the put option does not have to sell (it is optional, thats why it is called buying an option).  However, the seller of the put is required to make good on their promise to the buyer. The broker can require the seller of the put option to have a deposit, called margin, to help make sure that they can make good on the promise.   Profit... The buyer can profit from the put option if the stock price moves down substantially.   The buyer of the put option does not need to own the stock, he can sell the option to someone else.  If the buyer of the put option also owns the stock, the put option can be thought of like an insurance policy on the value of the stock.  The seller of the put option profits if the stock price stays the same or rises.  Basically, the seller comes out best if they can sell put options that no one ends up using by expiration day. A spread is an investment consisting of buying one option and selling another. Let's put bull and put and spread together with an example from Apple. So, if you believed Apple Inc. AAPL (currently 595.32) was going up or staying the same through JAN you could sell the 600 JAN put and buy the 550 put.  If the price rises beyond 600, your profit would be the difference in price of the puts.  Let's explore this a little deeper (prices from google finance 31 Oct 2012): Worst Case:  AAPL drops below 550.  The bull put spread investor owes (600-550)x100 shares = $5000 in JAN but received $2,035 for taking this risk. EDIT 2016: The ""worst case"" was the outcome in this example, the AAPL stock price on options expiry Jan 18, 2013 was about $500/share. Net profit = $2,035 - $5,000 = -$2965 = LOSS of $2965 Best Case:  AAPL stays above 600 on expiration day in JAN. Net Profit = $2,035 - 0 = $2035 Break Even:  If AAPL drops to 579.65, the value of the 600 JAN AAPL put sold will equal the $2,035 collected and the bull put spread investor will break even. Commissions have been ignored in this example." "When you buy a put option, you're buying the right to sell stock at the ""strike"" price. To understand why you have to pay separately for that, consider the other side of the transaction. If I agree to trade stock for money at above market rates, I need to make up the difference somewhere or face bankruptcy.  That risk of loss is what the option price is about. You might assume that means the market expects the price of AMD to fall to 8.01 from it's current price of 8.06 by the option expiration date. But that would also mean call options below the market price is worthless. But that's not quite true; people who price options need to factor in volatility, since things change with time. The price MIGHT fall, and traders need to account for that risk. So 1.99 roughly represents the probability of AMD rising to 10. There's probably some technical analysis one can do to the chain, but I don't see any abnormality of AMD here." One broker told me that I have to simply read the ask size and the bid size, seeing what the market makers are offering. This implies that my order would have to match that price exactly, which is unfortunate because options contract spreads can be WIDE. Also, if my planned position size is larger than the best bid/best ask, then I should break up the order, which is also unfortunate because most brokers charge a lot for options orders. A simple example - When looking at oil trading in different locations first I have some back of the envelop adjustments for the grade of oil, then look at storage costs (irrelevant in the case of electricity) and transport costs between two locations to see if physical players are actively arbing the spread.   No strong views on reading material in this specific area - Google, google scholar and amazon all have relevant material.  When it comes to your current problem, here are some questions to think about:  1. Is the power generated from the same commodity at location A and location W?   2. How has the spread changed in the past? Has trading location W actively hedged the worst cases of prices moves in location A?   3. Is it feasible to trade the commodity that location A generates the majority of its power from/how does that compare to electricity trading at location W as a hedge?  4. If hedging is really desirable, are you sure you can't do an illiquid over the counter hedge at location A? Paying a little bit more in the bid/ask for the hedge could be more desirable than trying to jump into a market you yourselves don't quite understand.  5. If your consultants come back with just some hedge ratios without discussing what drives the spreads between the two locations and where the spreads are currently be skeptical. "You're correct that the trading costs would be covered by the expense ratio. Just to be clear here, the expense ratio is static and doesn't change very often. It's set in such a way that the fund manager *expects* it to cover *all* of their operational costs. It's not some sort of slider that they move around with their costs.  I'm not familiar with any ETF providers doing agreements which cover rent and equipment (hedge funds do - see ""hedge fund hotels""). ETF providers do routinely enter into agreements with larger institutions that cover stuff like marketing. PowerShares, for a while, outsourced all of the management of the Qs to BNY and was responsible solely for marketing it themselves." I'm not exactly sure, but it may be due to liquidity preference.  SPY has a much higher volume (30d average of roughly 70m vs. 3.3m, 1.9m for IVV, VOO respectively), and similarly has a narrow bid ask spread of about 0.01 compared to 0.02 for the other two.  I could be wrong, but I'm going to leave this post up and look in to it later, I'm curious too. The difference is very consistent though, so it may be something in their methodology. "Mathematically it's arbitrary - you could just as easily use the bid or the midpoint as the denominator, so long as you're consistent when comparing securities.  So there's not a fundamental reason to use the ask.  The best argument I can come up with is that most analysis is done from the buy side, so looking at liquidity costs (meaning how much does the value drop instantaneously purely because of the bid-ask spread) when you buy a security would be more relevant by using the ask (purchase price) as the basis. Meaning, if a stock has a bid-ask range of $95-$100, if you buy the stock at $100 (the ask), you immediately ""lose"" 5% (5/100) of its value since you can only sell it for $95." A day is a long time and the rate is not the same all day. Some sources will report a close price that averages the bid and ask. Some sources will report a volume-weighted average. Some will report the last transaction price. Some will report a time-weighted average. Some will average the highest and lowest prices for the interval. Different marketplaces will also have slightly different prices because different traders are present at each marketplace. Usually, the documentation will explain what method they use and you can choose the source whose method makes the most sense for your application. Something like cost = a × avg_spreadb + c × volatilityd × (order_size/avg_volume)e. Different brokers have different formulas, and different trading patterns will have different coefficients. Thank you for replying. I'm not sure I totally follow though, aren't you totally at mercy of the liquidity in the stock? I guess I'm havinga hard time visualizing the value a human can add as opposed to say vwapping it or something. I can accept that you're right, just having a difficult time picturing it