user2846: I'd like to know how much the number changes depending on whether it's done using hardware or just in software. I'm curious how long you'd have to leave a computer mining bitcoins for it to pay for itself

user2306: It really depends on how much you're willing to spend.
Low end
Let's talk about the very smallest investment you could make for mining bitcoins: a Raspberry Pi. It can do 200,000 hashes per second, which might sound like a lot, but actually it's about 0.000001% of the total network hashrate.
Power: 3.75 W
Hashrate: 0.2 MHash/s
Cost: $35
(Note: this doesn't mean a RPi is useless for mining. You can use it as a host for your ASIC/FPGA miners.)
Mid range
Okay, CPU mining is bad, but what if we use GPU's?
Let's say we use a $162 graphics card, the 5830 from AMD. Let's throw in another $187 for case/motherboard/power supply. (Based off of this list I just threw together.)
Power: 175 W
Hashrate: 302 Mhash/s
Cost: $349
High end
Okay, let's get something specifically designed for bitcoin mining - the ModMiner Quad. It costs $1070, but it needs a host, so let's use a raspberry pi for that.
Power: 43 W
Hashrate: 800 Mhash/s
Cost: $1105
Ultra-high end
Okay, let's say that you have stupid amounts of money to use on mining hardware. Let's say that you can get a Bitforce Mini-Rig. (They are not being sold yet.) In fact, let's say that you can get 10 of them. This gives you 40% of the bitcoin hashrate.
Power: 15,000 W
Hashrate: 15,000,000 Mhash/s
Cost: $299,000
Sources for most of this
The lesson here is that you can spend any amount of money on bitcoin mining.

2846: <p>I'd like to know how much the number changes depending on whether it's done using hardware or just in software. I'm curious how long you'd have to leave a computer mining bitcoins for it to pay for itself</p>
 2306: <p>It really depends on how much you're willing to spend.</p>
<h2>Low end</h2>
<p>Let's talk about the very smallest investment you could make for mining bitcoins: a Raspberry Pi. It can do 200,000 hashes per second, which might sound like a lot, but actually it's about 0.000001% of the total network hashrate.</p>
<p>Power: 3.75 W<br />
Hashrate: 0.2 MHash/s<br />
Cost: $35</p>
<p>(Note: this doesn't mean a RPi is useless for mining. You can use it as a host for your ASIC/FPGA miners.)</p>
<h2>Mid range</h2>
<p>Okay, CPU mining is bad, but what if we use GPU's?</p>
<p>Let's say we use a $162 graphics card, the 5830 from AMD. Let's throw in another $187 for case/motherboard/power supply. (Based off of <a href="http://pcpartpicker.com/p/C7iy" rel="nofollow noreferrer">this list</a> I just threw together.)</p>
<p>Power: 175 W<br />
Hashrate: 302 Mhash/s<br />
Cost: $349</p>
<h2>High end</h2>
<p>Okay, let's get something specifically designed for bitcoin mining - the ModMiner Quad. It costs $1070, but it needs a host, so let's use a raspberry pi for that.</p>
<p>Power: 43 W<br />
Hashrate: 800 Mhash/s<br />
Cost: $1105</p>
<h2>Ultra-high end</h2>
<p>Okay, let's say that you have stupid amounts of money to use on mining hardware. Let's say that you can get a <a href="http://products.butterflylabs.com/1500gh-bitcoin-miner.html" rel="nofollow noreferrer">Bitforce Mini-Rig</a>. (They are not being sold yet.) In fact, let's say that you can get 10 of them. This gives you 40% of the bitcoin hashrate.</p>
<p>Power: 15,000 W<br />
Hashrate: 15,000,000 Mhash/s<br />
Cost: $299,000</p>
<p><a href="https://en.bitcoin.it/wiki/Mining_hardware_comparison" rel="nofollow noreferrer">Sources for most of this</a></p>
<p>The lesson here is that you can spend any amount of money on bitcoin mining.</p>
