On my larger organization, we have on average 20 to 30 *active* runners during business hours. Assuming 5 on the off-hours, my napkin math says it comes down to about 10 fully-utilized-runners per month, so about 864$/mo. For the size of my organization that is honestly totally acceptable.
This is assuming 0.002$ per minute of job being actively executed. If it turns out to be 0.002$ per minute of *runner being registered* on the control plane, it would increase quite a bit. We are still using the old HorizontalRunnerAutoscaler with actions-runner-controller, with quite a pool of prewarmed runners idling to pick up a job. It would be a strong reason to use the new RunnerScaleSet (to take advantage of the reactive webhook-based scaling) and keep a very lean pool of prewarmed runners.