I read this article:
We have paid 0.034 BTC for the 10 GH/s ... Yesterday we have received our first daily payout and it was exactly 0.00041279 BTC ... Doing a quick calculation has shown that if the Bitcoin network difficulty remains the same and the daily payout is also the same as the one we got the first day we are going to need a little over 82 days to get back what we have spent for the mining contract.
In this case BTC per GH/s is 0.00041279/10 BTC/(GH/s). All of this is in one day, but for simplicity I removed day from the units.
Will buying more GH/s increase BTC per GH/s (in one day)? Since there is more power solving the problem, and there would be more probability to solve the problems?
How could I calculate decrease of BTC/day vs time with a fixed hashing speed?