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.

Definitely, if you can cut costs, do 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!