Comment by ZeroGravitas

Not OP but Lazard has published the marginal cost of existing, paid off nuclear at 29 per MWh. Someone in this discussion who sounds pro-nuclear shared almost the exact same number.

Lazard suggests the low end for new nuclear in the US is 131 per MWH (over a 60-80 year life at 97% capacity factor).

So, assuming that old nuclear cost the same amount as new nuclear that's about 23%. If you assume nuclear is more expensive today, which everyone appears to agree on, then you easily get to 1/3rd of the cost being the marginal cost of running nuclear when the original build is paid off (more if a substantial refit is needed, see France).