And not much compared to a bug in prod or waiting 3 weeks for a release (the value proposition of CI/CD)

Still, saving cloud spend by making the build faster or using self hosted runners is probably worth it.

An additional several thousand dollars per month for any moderately-sized team, especially if you've got a bunch of microservices, sounds very expensive to me, and would probably get axed quickly by any devops team I can think of that's actively working on reducing their costs.

I mean if you're paying 5+ digits a month on CI it starts being reasonable to say "maybe I should do things to lower the absolute cost of this". 5 dollars time a big enough number is real money!