user6924: I am a beginner looking into Bitcoin mining. I don't want to waste time (or electricity) so I started looking into hardware mining rigs, for example:
Saturn
priced at $3000 USD, 
275GH/s, consumes ~400Watt
(or even Monarch from butterfly labs which claims 600GH/sec @ 350W, priced $4700)
I put these numbers + my local electricity price + current difficulty into a decent calculator, 
and I get >7,000 $USD profit for 3 months time frame (or >18,000$ for the Monarch). Sounds a bit too good to be true.
I know BTC/USD rate might fluctuate, and that the miner will produce heat, but still. - Where's the catch/mistake, if at all?

user5757: Keep in mind that difficulty also evolves over time: the more hashes the network produces, the higher the difficulty becomes.  As of this writing the difficulty is 148,819,199.  By my computation, each additional 275GH/s rig that joins the network will increase the difficulty by about 40,000; if 4,000 of them are sold, the difficulty will more than double (not counting all the competing mining rigs that may join the network).  And of course, in a few more months, there may be even more powerful rigs available.  So you should expect difficulty to rise, perhaps sharply, and this will decrease your profitability over time.

6924: <p>I am a beginner looking into Bitcoin mining. I don't want to waste time (or electricity) so I started looking into hardware mining rigs, for example:
<a href="https://www.kncminer.com/categories/miners" rel="nofollow">Saturn</a></p>

<p>priced at $3000 USD, 
275GH/s, consumes ~400Watt
(or even Monarch from butterfly labs which claims 600GH/sec @ 350W, priced $4700)</p>

<p>I put these numbers + my local electricity price + current difficulty into a decent <a href="http://blockchained.com/profit/index.php" rel="nofollow">calculator</a>, 
and I get >7,000 $USD profit for 3 months time frame (or >18,000$ for the Monarch). Sounds a bit too good to be true.</p>

<p>I know BTC/USD rate might fluctuate, and that the miner will produce heat, but still. - Where's the catch/mistake, if at all?</p>
 5757: <p>Keep in mind that <em>difficulty</em> also evolves over time: the more hashes the network produces, the higher the difficulty becomes.  As of this writing the difficulty is 148,819,199.  By my computation, each additional 275GH/s rig that joins the network will increase the difficulty by about 40,000; if 4,000 of them are sold, the difficulty will more than double (not counting all the competing mining rigs that may join the network).  And of course, in a few more months, there may be even more powerful rigs available.  So you should expect difficulty to rise, perhaps sharply, and this will decrease your profitability over time.</p>
