Being honest, knocking one refactor after another is super intense and we would burn out quickly. To me refactoring/reading/reviewing a code that you have never seen is one of the hardest thing to do for a software developer. If I have 4 such projects a month, I'd just hire more people and allow them to rest. Also invest more into internal tooling to speed things up would be good solution to remove some burden. I think $10,000 for refactor is a fair price.

We also have other gigs and ongoing project, where we can rest a bit.