It is hard to say because you really need to factor the R&D and the marketing.
Once the production is in full motion and the yield are goods, it probably cost them a fraction of what you pay for it.
GDDR4 is substantially more expensive then GDDR3, I don't know by how much but it does make for a big portion of the overall price of the card. I find it hard to believe that a R600 would come with 1 gig of GDDR4 for that reason!
The flagship of both company is always 'overpriced', you pay a premium for diminishing returns. But they know some people MUST have the cutting edge so the capitalize on it. That's also on of the main reason you see the Highend always coming before the lower end of a new generation.