Can I stop a workflow running up a large bill?

Yes. In settings you set a monthly limit in dollars, either on a single workflow or across everything you run. The month rolls over in a timezone you choose, so the boundary lines up with your accounting rather than with UTC.

The limit is checked when a run is admitted, never in the middle of one. A run that has already started finishes. That keeps the guarantee simple: hitting a cap never leaves work half done, and it never leaves an external system in a state nobody planned for.

What happens to blocked work depends on who asked for it. Anything unattended, a schedule firing or an event arriving, parks: the job waits, labelled with the limit that stopped it, and starts on its own at the beginning of the next month. Work you started by hand fails straight away with a message naming the limit, because you are standing right there and can decide what to do.

You get one email per limit per month rather than one per blocked run.

Raise the limit or remove it and exactly the work parked on that limit resumes. Nothing else is dragged along with it.