How much utilization do you have? For low scale, it's hard to beat GitHub Actions as they offer free runners for public repos and include a bunch of free hours for private repos.

Once you start paying for it, GitHub Actions runners are very expensive. I've used both Jenkins and GitLab before to self-host CI/CD, and you save so much using on-demand (or at higher scale, reserved) cloud instances. I do freelance DevOps work and I've helped clients with these sorts of challenges.