This isn't a real offer, but I think $175-$200 would be fair from a miner's perspective. I see 5830's on the forums for about $85-$95, and a 6870 is just a slightly revamped 5830, uses the same VLIW5 architecture and 1120 shaders per GPU. This card has 2, so double the price, plus a premium for being 1 PCI-E slot and a "newer" generation. It has a degree of resale value. Gamers might into it for $210-240 I'm guessing though.
In reality though, I -could- be far from the mark. I've only seen a handful of 6870x2, so certain people may hold value in it's rarity. Looking on ebay and google shopping, the lowest price is around $350, and I can buy Radeon 5970's for $250, which are MUCH more powerful cards.
Newegg were selling these brand new for $160 a while back, if I was US-based I'd have bought a dozen
But yeah, on ebay and the like you can get $300 or equivalent without too much difficulty. Certainly wouldn't expect it to go below $250.
I had one for a little while and it ran at ~585Mhash/s, but with much poorer efficiency than a 7970 doing the same job