@WilliamWatts
I do see what you're trying to say.
As I linked earlier, there's no conclusive evidence that games perform better under DX11 mode than under DX9. We'll need more DX11 games to come out to tell.
You are comparing the current price of the 4870 to the 5770, however just before the 5770 released you could find 4870s at $125 most of the time. The 4850, 4890, GTX260, GTX275, GTX285 and GTX295 have all risen in price by a large amount since the 5xx release - 5xxx has made the price of a given level of performance rise.