i used the calculator at VNBitcoin (
http://www.vnbitcoin.org/bitcoincalculator.php). It shows that, based on the current price of 0.0083BTC/GH if the BTC rate/USD stays at $623/BTC, and difficulty increases by 16.5% each time, it'll take about 9 months to break even. If BTC goes up to, say, $800/BTC you'll break even after 4 months. the difficulty increases at, say, 20% (with BTC at $623), it'll take more than a year.
so - you're basically gambling that:
- BTC rate will go up, or
- difficulty rate will not go up significantly.
you may well be better just buying coins.