I think what you are missing (and its not just you doing this) is making an assumption that price and performance should have some direct relationship....i.e. 8% better performance should be about 8% higher in price.
In reality, and in almost anything in a market economy, price is a function of many complex factors. It is true for cars, food, and video cards. It is not going to change. The price of the 5850, in theory, is set at the optimal point to maximize profits for AMD and its board partners. If it is the case they have little inventory and are selling most or all of the units they are making at the higher price, they do not need a price cut. You may want one, but it is not a good business choice for them. Then, each time period later, they have to decide whether they could make more money by lowering the price (by selling more units) or not. They clearly believe that the current price points are profit maximizing or they would be coming down faster.
Also, and Wrekage talks about this below albeit in his usual trollish ways, Nvidia and AMD both attempt to develop cards that have extra features that help them differentiate their cards from each other. If the cards merely did the *exact* same thing, then all they would have to compete on is price, and that is usually a losing battle resulting in one of the companies going bankrupt.
For example. I am in the market for a new video card. I have a 4850 from AMD. It has been a great card for me for the past few years. However it is starting to show its age. I also have 2 monitors I use extensively for work purposes. I would like to add a 3rd monitor to my office setup. I do not have an SLI motherboard. So, if I want mainstream or high mainstream graphics card performance and 3 monitor support I have to decide, do I buy a new motherboard and 2x Nvidia 460's or higher, or do I buy 1x 5850 or 5870? Since I do not need game on 3 monitors, just the larger center 30" monitor, the cost issue is strongly in favor of AMD on this one.
Point being is that their are some reasons someone should pay a higher price for AMD 5850 over a 460, with one being the case above. It makes more sense for me to pay a $65 premium to get what I want, even though the performance will be almost the same, because by spending the $65, I am actually savings hundreds by not needing an SLI board and 2x Nvidia cards.
So moral of the story, price/performance is dictated by many different market forces and many situations exist where it is easily justified for a product to be most costly for the same performance. It may not be true even some of the time, but those situations certainly exist.