Ok.

You could:

- host your own set of static runners on AWS -- which, have a fixed monthly cost.

- pay a provider for hosted runners -- most providers bill in CI Minutes. So you will run out of minutes if jobs run amok, not run up your bill.

- Set up auto-scaling runners that ebb and flow based on demand. This case is the one that represents the risk you are describing of an unexpected bill increase.

2/3 cases of "paying someone else for CI compute" are just as predictable as your solution cost-wise. Yours could be cheaper, but the risk of "unexpected bill increase" is not really there.