We were hemorrhaging money on self-hosted runners for our mono-repo. 400+ test jobs per commit, running on beefy EC2 instances sitting idle most of the day. Switched to matrix strategy and cut costs by 70%.

The trick: chunk your matrix by test type, not by every conceivable permutation. We do node-version x os x test-suite instead of node-version x os x test-suite x database x cache-backend. Parallelizes like crazy without exploding your job count.

strategy:
  matrix:
    node: [18, 20, 22]
    os: [ubuntu-latest, macos-latest]
    suite: [unit, integration, e2e]

Self-hosted is only worth it if you need specific hardware or have compliance lockdown. Otherwise you're just paying for DevOps overhead. GHA scales to zero. Your runners don't.