And in about 10 days, if the difficulty jumps up another 50%, it'll reduce that 0.636 BTC down by a factor of 1/1.5 = 0.666, which would make it 0.4236 BTC.  10 days after that, another 50% jump would reduce it to 0.2821 BTC, then 0.1879 BTC, etc.  This is why you should never estimate earnings with no difficulty increases.  50% increases every 10 days is a lot but it hasn't really been letting up.  33% increases every 10 days is probably more reasonable but even at that slower rate, BTC needs to be at least $30 to possibly break even on new rigs.