That's probably enough for 99% of people, and if you're highly motivated, you could make that trigger an SNS notification that trips a circuit breaker.
No, that's really not good enough. I don't want to need to be "highly motivated" in order to set a limit, I want to say this thing cannot use more than this many dollars each month, no conditions no exceptions no questions. If I make a fun little side project and it hits the front page of HN, I don't want to quibble about whether I cut it off in time or some hacked together little script turns things off correctly, I want it capped.
That's probably enough for 99% of people, and if you're highly motivated, you could make that trigger an SNS notification that trips a circuit breaker.