The costs are not really due to regulations. They are because the construction industry is not exactly known for quality.

We know how to design safe nuclear reactors. We just can't build them cost-effectively, because there is always some subsubsubsubcontractor that doesn't bother doing things by the book. Then an inspector notices that something is wrong and orders it dismantled and rebuilt. And this will be iterated until everyone manages to do the right thing at the same time.