The best I can say is to check Ebay and see what one is going for. I see the 6870s on Newegg (about the same power based on Gigaflops based on info given on Wikipedia) going for about $190 USD (a quick mental average, with odd balls (like those going for $400) being left out). You might only get around $125 to $175 for that card if you sell it, given this, depending on condition, overclocking, usage, and various other factors.