About $20-30, so I guess you're right there.
How much is the estimated performance difference between the two though?
I'm on a 7870 myself.
Well, 7850 CF is generally slightly ahead of a 7970 GHz Edition in average gaming performance and a 7870 is a good 25-35% faster than a 7850, so I'd expect two 7870s to have a worth-while lead on the 7970 GHz Edition. Unfortunately, I don't think that there are any such comparisons with current drivers since I haven't found nor even heard of them, so I can't provide benchmarks that aren't already outdated.
Cheers for the comments and suggestions guys what about if swap the 2 x 7870's for a Sapphire ATI HD 7970 OC 3GB 384-bit GDDR5, which will save me ~$70 and then add a OCZ 256G Vertex 4 Series SSD to put me at about $1300?