take the current difficulty and put it into the generic standard bitcoin generation calculator to find out how long it will take you on average to create a block of 32 solid coins. Then I took the number of solid coins that I would make in a day and multiplied it by the exchange value at scexchange.bitparking.com and found out that 4 GH/s would produce about 1.7 BTC which is less than I would make if I was mining at say deepbit.net.

There ya go!