I think that's not really an issue though is it? If you say "never charge me more than $100" they can a) ensure they never charge you more than $100 and b) work

I think that's not really an issue though is it? If you say "never charge me more than $100" they can a) ensure they never charge you more than $100 and b) work to optimize their own systems so that they cut you off as close to $100 as humanly possible. In the beginning they might eat some costs since it takes them a day to catch it, but they could work over time to bring that down. And it's not like it's costing GCP/AWS/Azure "sticker price" to provide their services.