regardless, the die sizes are similar. nvidia has lower costs but amd has significantly higher asp's. right now amd is still in the driver's seat, but nvidia is at least on the bus finally.
I think that gtx 460 is nvidia's 3870. We all know what came after that, and the fermi architecture looks to have an even stronger starting point. however, amd isn't exactly resting on its laurels like nvidia did with g92 and its 14 rereleased, rebadged cards. hopefully SI and fermi II are able to reignite a new graphics war for us all.
The thing you are missing here is that while margins are very important to know in regard to whether a product is profitable, you have to look at Nvidia's ability to spread its R & D costs of its product family out over multiple chips.
Somewhere is a number ($$$$$$$$$ most likely) that represents the R & D to develop and product the Fermi chips that are out. Ideally, you can then spread this cost out over lots of chips at different price points. If Nvidia spent $100 million developing Fermi and sold 100 million chips, the R & D cost per chip would be $1, making the rest of the margin profit. But, if they only sell 10 million chips, R & D eats away $10 per chip and so on.
So we cant say that Nvidia's costs are higher or lower until, as I stated previously, we know the margins on the chips, and the rest of the costs that went into production, R & D etc.
I would also say that it is unlikely that Nvidia has lower costs on a similar sized die, given how behind they were in releasing Fermi. As has been previously reported, ATI spent quite a bit more time working on process technology issues than Nvidia (although I dont know this to be a fact, it was just talked about a lot on this forum) and has quite a bit more time on the process node in actual product before Fermi started to even sample. Usually this should translate into ATI having better control of defects, production issues etc...leading to lower production costs. But again, that is just my speculation given the normal tendencies in semiconductor manufacturing.