The projected difficulty for late February 2014 is somewhere around 3 billion on the low side and something like 5 - 6 billion on the high side. 312 Ghs at that difficulty and the current exchange rate for bitcoins would yield about $425 per week with the yield dropping about 15 - 20 % every 12 days or so. The yield for 6 billion difficulty would be about $200 per week. That weekly yield would give about $20,000 or $10,000 earnings WITHOUT any further increase in difficulty which will never happen.

Morris