One-time income with recurring costs: how to mantain an app in the cloud

Question

You have an idea of a software/app/webapp/website that you would like to spend some effort and implement. Your idea uses a cloud system to keep things up and you think you could earn some money by distributing your product and charging a small one-time fee. But there is a problem: the first thing you realize is that one day that user using your system may eat all the money he gave you with his indirect recurring little spending in the cloud. How would you do the math to find out if your idea was feasible?