Building a subscription API?

Hi!

I'm curious who here is building subscription APIs. I'm experimenting with the idea of a service that handles the work of monitoring and charging accordingly for API usage. My idea is for this to be really easy to integrate and to provide dashboards for API owners and their clients to monitor their usage and handle subscriptions. Do solutions like this already exist?

I'm curious to hear from anyone who has built, or is currently building, something like this into their APIs. How are you charging for usage? Is it by the number of API calls? By the amount of data stored/consumed? Some sort of credit system? I'd love to hear all about it!