Whoops, I managed to misread 6990... In that case add... $100-150 to each of my prices (being conservative, I may not be the best person to decide this since I tend to lowball).
considering new 280x goes for 250ish, so 2x is 500, id charge no more than 500$ the only thing giving this value is the fact that its a dual GPU card, so something "special"