I wonder what the internal cost is for TSMC to do a wafer is?

The variable cost if you will: The cost of materials, electricity, staff etc.? Probably a very small amount.

The big cost for TSMC is the fixed cost of the expensive equipment and the massive R&D in developing the process. However the amount is interesting since if you wanted to spend a massive billions of dollars and setup a competitor to TSMC, you know that TSMC would be able to compete with you on price all the way down to that point.