>Besides, if they can trigger alerts at a particular spend then they should be able to create a limit.
That's not really true. The alerts happen when the billing is re-calculated (periodically) and you've exceed a predefined, not when you hit that exact threshold.
https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitori...
>When you enable the monitoring of estimated charges for your AWS account, the estimated charges are calculated and sent several times daily to CloudWatch as metric data.
Real time billing is actually a Hard Problem to solve.