What makes is it so expensive?

Small reactors miss out economies of scale that large reactors get access to. Halving the size of a pipe doesn’t halve the cost.

In the case of our experimental reactors, a large part as I understand it is that they were fooling around in the 60s. Thus a lot of "meh just put it over there" scenarios and such, without much consideration of the consequences.

Another point is that Norway has no way to process the leftover materials, and currently nowhere to store the processed materials.

So we need to ship the highly radioactive stuff abroad, and build a storage facility for when it's processed.