How much more would it cost to make nuclear power plants underground?

Compared to the cost of a plant as a whole, I don' think it would be much. Guessing 1.5X. If there is much groundwater, it might take sealing and pumps to keep the plant dry, and in a "seismic event" some places might fluidize, and the plant might actually float up. It could also make repair / replacement of some large parts more difficult.