author here - we are using spot instances on GCP for executing the pipeline which at 10 hrs/day usage makes us $1 in profit/month :)
What sort of spec machine (or kubernetes pod) are you providing for that?
Product wise we don't want the user to worry about machine resources as long as CI executes within reasonable time.
Out of interest, have you modelled spot instance restarts into your costs? How about spot instance pricing going up?
Spot instances do take upto ~2 mins to come up so probably a turbo add-on that devs will feel happy paying for or please feel free to suggest.