But hurry up because in the future you will make $0.75/per GPU then $0.50/per GPU
We're in the bitcoin mining section, so i presume you're talking about sha256d hashing... I made a rather big effort to search google for sha256d benchmarks using GPU's.
https://en.bitcoin.it/wiki/Non-specialized_hardware_comparison lists speeds lower than 1 Gh/s for single GPU's, and speeds lower than 3 Gh/s for "farms" of 6 GPU's. It is reported this farm draws a whopping 1200 Watts, not including the controller (i presume).
If you get the 6 GPU's + the PC for free, and you pay 1 cent/Kwu on a 0% fee pool, running this "farm" would make you:
http://www.coinwarz.com/calculators/bitcoin-mining-calculator/?h=3&p=1200&pc=0.01&pf=0.00&d=422170566883.83700000&r=12.50000000&er=1045.02020000&hc=0.00
A negative profit of $104/year. If you got the power for free, it would make 70 cents a year at current diff, block reward and BTC price... So in the best case scenario (free hardware, free power, steady diff, steady price, 0% fee pool), you could potentially make $0.00186 /day mining a 6 GPU farm... As soon as you pay 1 cent/Kwu, mining bitcoin using a GPU will cost you about 30 cents/day in electricity while generating $0.00186 in profits.
These days I think its common sense when you mention a GPU you aren't talking about Litecoin or Bitcoin mining.
Buy GPU
Mine ZEC/ETH/XMR
Sell ZEC/ETH/XMR for BTC
I am pretty sure most GPU miners sell their alt-coin for BTC.