True, but we've built tokamaks and we're building ITER, which so far has an estimated price of between $45 billion and $65 billion.

Now of course that's a research reactor full of experiments and instrumentation that wouldn't be part of a normal power plant, but given current experience that I think we can expect we won't suddenly knock down the cost to $100M. It's going to be somewhere in the billions. And we have expectations of that DEMO is going to make 750MWe.

We can then plug those estimates into the calculator and basically figure out how cheap and how powerful a fusion reactor has to be for it to make economical sense.

Part of that cost is from ITER being so huge, which is because they use obsolete superconductors. CFS is doing the same thing in a reactor a tenth as big, using newer superconductors that support stronger magnetic fields. Tokamak output scales with the fourth power of magnetic field strength, and the square of size.

The size and also the complicated governance have made ITER very slow to build, which also increases expense. The JET tokamak is about the size of the reactor CFS is building, and JET was built in a year for the reactor itself, plus three years before that for the building they put it in.

I think a lot of the cost is custom parts. Standardization and economy of scale would bring the price down quite a bit.