I don't think AWS's billing system is robust enough for that. In pure AWS fashion you have to create a billing alarm, which pushed an event to SNS which triggers a lambda to shut down your stuff but it is possible. The catch is that billing metrics are estimates and alarms might be delayed.
Yup, seems like they have a lot "eventually consistent" batch processing going on. Certain things like CloudTrail have 4-6 hour lags in billing (unless it's improved in the last year)
It'd be a huge engineering effort to make something instantaneous--I think the closest thing they have to such a system is whatever they use for rate limiting or IAM.
I'm guessing there's a pretty high overhead to trying to do realtime instead of batching
AWS oopsies suck but I think their billing system is pretty robust compared to lots of usage based billing systems (like, say, utilities)
