That doesn't sound like a great experience for users.
1. They don't directly know what actions are causing how many API calls, so it is unclear what they are buying.
2. It creates the incentive to use the service less to save money.
3. They aren't directly in control of what API calls the app makes, so there is some disconnect. (What if the developer decides to preload some info because it creates a better experience?)
Generally the user should be purchasing based on the value it offers them, not the cost to provide the service.
