I haven't paid a dime over $250 for any of the 7950s I own. at 650khash (that's what I'm getting), that's 2.6, I can get double or more hashrate per mobo AND per watt of electricity, not to mention MUCH better resale/repurpose value.

I wouldn't buy anything slower than a 6970 for mining, and I'd have to be getting them dirt cheap.