I am sure Blacksmith is the easy path if you don't want to do any extra work and I am also sure their solution will scale much better as this is what they do full-time.

However, I can share what we did to ease on our GitHub Actions bills if it helps.

Effectively we have our own runners hooked so that a job is scheduled, a runner picks it up and goes with it. We still use GitHub Actions but our monthly bill is now flat because we pay for a server. It is about 6x cheaper if not more.

The solution is not open source but it boils down to a Go service that orchestrates firecracker vms. All the vms are pre-warmed so there is always a fresh supply of workers to pick jobs of various sizes.

It is basic and it works. We have not had any issues since deployed.

The runners can be anything from 1 cpu 2 GB to 64GB 8 cpus. We can add more worker types in a config file.

I am not exaggerating when I say that we used to pay 1000s per month for this. Now our bills are in the range of a few hundred. Other dev boxes are done in the same way.