You can use Kubernetes with Kaniko or Docker-in-Docker or rootless Docker for building stuff too. I think that's much cheaper in an org context. The amount of CI we run, we have saved countless dollars.
Set of fat buildkit instances per build shape served on a spot instances autoscaled down to 0 with KEDA and Karpenter is pretty close to as optimal of a setup as one can get
Set of fat buildkit instances per build shape served on a spot instances autoscaled down to 0 with KEDA and Karpenter is pretty close to as optimal of a setup as one can get