The difference is that people pay 100$ more for a 5870 than a 5850 because the 5870 is 10-15% BETTER.
2x GTX 460 is anywhere from 5-10% BETTER than 1x GTX 480 and costs 60-100$ less
People will still buy the GTX480 with the reasoning that they can add another one later for SLI. Pricing on GPU´s has never been linear with performance.
HD5970 = $700 same performance 2xHD5850 in Crossfire = $600. Should ATI pay back every one $100 that bought HD5970.