At what price and model?

7770s' cost $80 with the sale of the crap game = 170ish M/hash  2.125 M/Hash per $1
7950s' cost $270 with the sale of game = 580ish M/hash  2.148 M/hash per $1

BUT PSUs' have dramatic price jumps over 1000 watts. So I believe mixing the 2 cards is best. Also mixing them allows for more rebates to be valid. Also in theory selling them is easier since its targets multiple markets