[JustSteveKing]

This shouldn't be charged per API call, you should look at an average of API calls per person - and create a figure 20% above that of what you want to earn.

Alternatively, charge x per month - with a cap of a certain amount of API calls.

The problem with charging per API call is that what do you class as an API call? What happens if a network degrades and the response hangs? Should the end user pay for this? What if they get a 404, or a 401/403? Should they be charged for these?

As your application stores customer data that they own, I could price based on how many records they are storing - not how often they want to access their own data.