We have really good insight into week-ahead curves for wind and solar, and high quality prediction for day-ahead production, so it wouldn't need to be ready "at any time" like its usage would be a surprise. And you incentivize it like anything else, providing either a $/kwh figure that makes it economical for them to capture their costs or subsidize the construction and standby nature of them like we currently do with black start generators (https://www.kut.org/energy-environment/2021-08-05/if-the-tex...).

It's hard to make a reliable cost prediction comparing nuclear vs. wind/solar + batteries since we don't know how to build nuclear any more.

Vogtle 3/4 are going to cost maybe $40 billion when all's said and done? With OpEx, you get to something like $0.18/kwh. That's more than 5x the cost of unsubsidized wind or solar installations which would buy you a bunch of storage.