While decommissioning is indeed expensive and as far as I'm aware still mostly based on estimates since there's little experience with the full process, I don't think this statement is correct.

What's really, really expensive is building the plant in such a manner that it is both secure and maintainable.

The problem with securing it is that a critical event is so damn expensive (cf. Fukushima) that you need to secure it against all imaginable problems - and as you can imagine, that list is growing over time. And still, you can't buy insurance for a plant. So the government carries the bulk of the risk.