I really wish cloud providers would allow users to set a hard budget that just stops the service if you exceed a threshold. I got a surprise bill from AWS this month (fortunately orders of magnitude less than this, but still ~15x my usual) and am thinking of moving from Lambda to a VPS just so the possibility of this doesn't keep me up at night.
I think there'd be two big issues - (1) what is reasonable behaviour when you reach your budget, and (2) the consistency of billing. On (1), if you get to say the 14th of the month and run out of budget, what would be the expected behaviour? - shut down all your services? delete all your data so that's not incurring a charge too? It'd probably have to be configurable on a service by service basis, and that's an awful…
what about devs too scared to put their CC no in the till coz they've heard horror stories about being overcharged for mistakes? Just recently had to sort out a feature that a junior dev created using openstreetmaps coz they didn't want to be on the hook by putting their card into google maps... It was for an internal portal, usage wasn't even high enough to hit the monthly free tier!
Hour 1 experience also matters.