thats b/c agp mobo's are outdated. You shouldnt be buying them anymore. Obviously they drop the prices when the newer technology comes out. ANd YES PCIe cards ARE cheaper than AGP equivalents. Do a search man.
Actually, it depends. Since the 6600 GT is native PCI-E, it would cost more to convert it to AGP. But since the 6800's came out native AGP, it would cost more to convert it to PCI-E. BTW AGP isn't outdated. It's perfectly fine.
