Japan was able to start construction on a new nuclear reactor and have its construction (and start commercial service) with-in five years, and did so every year from the mid-1980s to the early-2000s:

* https://en.wikipedia.org/wiki/List_of_commercial_nuclear_rea...

The first unit you build will be expensive, the second less so, the third even less. Economies of scale work just as well for nuclear plants as they do for widgets: Vogtle Unit 3 was expensive AF, Unit 4 cost 30% less.

Turns out if you don't know what you're doing, things can get be expensive because you're learning lessons along the way, but if/once you do know what you're doing costs drop.