Wrong. PCI-E is much more expensive with Nivida is because of SLI. Also your supply and demand analogy is flawed, because it would imply that AGP cards would be in short supply, therefore increasing the price, but lets not get into that.
Its because of SLI, stupid Nivida, grarrrrrrr.
6600GT PCI-E for sale/Trade. Check the FS/FT forums on tech-forums for details.