Theres some truth in this....
Remember when the top end single GPU card cost £400-£500?
Suddenly we have 4850's for £130!
To be honest, I actually think we have AMD to thank for this. Blame it on AMD's lack of success with getting the R600 to perform as well as it should have if you will, but the launch of their highest end card at less than £300 forced Nvidia to drop prices. They've only being going down from there, until we have ATI's top performing single GPU solution selling for £180, a far cry from the £400 it would normally have cost (back in the old days). Which, of course, forced Nvidia to drop prices, something even Nvidia fans should be grateful for.
I wouldnt be surprised if the truth is something along the lines of ATI and Nvidia had an agreement to keep the high end cost a lot. ATI couldnt compete with a fixed price because their cards didnt really perform as well, so although the agreement got them much bigger margins, it was hurting their competitiveness. So they broke the agreement, dropped prices, and started competing on their own terms.