How are stacks priced if they are constantly being created/destroyed? I want to make sure that each stack created doesn’t incur a $30 charge. E.g. Doing a stack for each PR but always have < 10 stacks at any given time since PR’s get closed.

We only charge for the high watermark, so if you never have >10 simultaneously, you don't need to pay for anything beyond 10. Further, note that right now we only have "soft limits" and you pay for a certain number up front, so you'll never get surprised if you burst beyond that number temporarily.