...and that's why CloudWatch allows you to set billing alerts. You could even route a billing alerts (via SNS) back to a Lambda function, use it to set a flag,

...and that's why CloudWatch allows you to set billing alerts. You could even route a billing alerts (via SNS) back to a Lambda function, use it to set a flag, and run in a reduced capacity mode if this is a concern. I always tell my audiences (I work for Amazon) that it should be possible to build a system where you know the actual cost of every web page you generate and every request you serve, and to make sure that each one has the ability to earn its keep (by ultimately driving revenue).