I think they get to a price by a combination of things. I think the first thing that each does though is compare their card to the competition, see where that card is priced, then adjust the overall level of costs for the card. They probably arrive at the base GPU/VPU price from that comparison, and then create the card using components that will allow the manufacturer to achieve that price point yet still make adequate margins. NVIDIA does a lot of balancing on their GPU costs so they don't bankrupt their manufacturers, as well as insure that they make good margins on their cards.
In the case of the 6600 series, NVIDIA really is sitting in a great position with that product. It has been out for more than a year, development costs are pretty much already paid for by how much they have sold to this point, and the cost of manufacturing the GPU itself has gone down quite a bit over the past year. We also see the prices on components go down, and DDR-2 memory is a great example. Definitely not the same stuff as was released with the FX 5800 or the 9800 Pro 256, but dirt cheap for good densities.
As for the high end, I would say that the base cost for a high end card such as the X1800 XT or 7800 GTX 512 is quite a bit higher than you would expect. A manufacturers cost on such a board would probably be around $450 once components and man-hours are accounted for. The manufacturer then needs to make some margin off of the card, and that card then goes to the wholesalers, which then take another chunk, and then onto the retailer (who also needs their cut). Admitedly, the 7800 GTX 512 priced at $749 is somewhat crazy, but we live in a world of supply and demand. NVIDIA priced the card so that everyone could have good margins at $649, but with the price at $749 everyone is getting a bit fatter. Still, how many video cards does Newegg sell vs. how many total 7800 512's they have pushed through? While margins are nice, the rest of their vid card business simply buries it. If there were no demand for the 512's, then they would be sitting on shelves and the retailer would lower the price to get their money moving.
So, while it is probably quite complex to take all of this into consideration, having a competitor and having already established price points, it makes it a lot easier. Now all these guys have to do is make sure their chip R&D doesn't go crazy, and they can find good enough and cheap enough components to make sure everyone in the supply chain has good margins.