A complicated solution is not what people are really asking for though.

What I and I expect most people want is a cap which then spins down services when they reach that cap. Nobody is going to care if the cap is set to $1000 and the final bill is $1,020. The problem being solved for is not wanting to have to ever worry about missing an alert and waking up to a bill that is a factor or two beyond expectations. I can afford my bill being 10% or even 40% above my expectation. I can't afford my bill being 500% off.