Roughly estimation/calculation
At 365mm2 we could have ~156 dies per wafer with 100% yields.
Cayman will produce roughly the same number per wafer.
With $5000 per wafer each die will cost 5000/156 = ~$32.00
Lets assume TSMC asks 6000 per wafer, that will translate to 6000/156 = ~$38,5
I will assume Tahiti has at least 60% yields now, that will make ~94 dies per wafer. 6000/94 = ~64 per die
Let’s assume Cayman has 85% yields and at 389mm2 will produce ~133 dies per wafer. 5000/133 = ~$38.00 per die.
64-38 = $26 die price difference between Cayman and Tahiti
Now lets assume 1GB GDDR-5 is $25 more,
26+ 25 = $51.00
Well, even with low yields Tahiti 3GB only costs ~$50 more than Cayman
Edit: Just to add, AMD will purchase 1GB mach cheaper than $25, but wanted to inflate the price a little bit.