Now imagine that we don’t have any practice building one of these things because people have been scared of building them for approximately 40 years, so the last person to make one is 70-90.
Now imagine that the failure case for this thing is a disaster that causes an expensive cleanup in the surrounding area, so no one will insure the project against that unless you pay an astronomical amount. Now add in that you’re using a “new” design that’s never been tested, and you have to follow a bunch of government regulations that didn’t even exist when the last comparable reactor was built.
Stack up all these things and you get a lot of uncertainty about project scope, uncertainty about financing, and uncertainty about whether it will work.
This means that to ballpark what it was cost, it will necessarily be expensive because of the risk everyone is taking in producing it.