Why do you think fission plants are expensive? Do you think it's the pumps and turbines and concrete?

Pro tip: it's not. It's because there is millions of man-hours of regulatory burden attached to every decision, to every bolt, to every instrument or valve installed.

There is a reason for all that regulatory burden, of course. It's the release of long lived and deadly radiation from a meltdown. If it wasn't for that regulation building a nuke plant would actually be quite inexpensive, relative to current costs- On the scale of a hydro dam.

Fusion has none of those costs because it has none of the same dangers. It's a wildly different problem with wildly different cost basis. The expensive part is research. Once that's done that cost is gone.