Hard to say, since the effective cost of each kind of GPU varies wildly depending on yield. I'd wager somewhere between $20 at the low end and $50-100 at the top. It's usually an exponential increase in effective price as you go up in speed, because only a few chips will be able to run at, for instance, X850XT speeds with 16 pipes, whereas a lot more can run at X800Pro speeds with 12 pipes, and a whole lot more than that will run at X800SE speeds with 8 pipes.
It's also pretty unrealistic to not factor in R&D and one-time setup costs. If the cost to the company is $10 million + $20 per GPU, the effective price is a LOT more than $20/GPU until you've cranked out millions of them.