"Nuclear modernization" in this context is not some effort to make nuclear power plants safer or more efficient. It is a couple hundred billion dollars to replace 450 nuclear missiles with 600 other nuclear missiles. Something like $500 million per operational missile.

So the cost is presumably something like 2 missiles. But don't worry, the project is already years behind schedule. The first $75 billion bought us exactly 0 operational missiles in the 15 years since the contract was awarded to the only bidder.