> If you're using the API yourself it means the cost of building is complete
Opening up an internal API with a know client that you own to the whole world introduces more complexity around authentication, rate limiting, traffic inspection, etc, that OP probably doesn't need to worry about now.
It is my data - if I am paying for a service already the idea of paying more to access my data differently is unattractive.
If you're using the API yourself it means the cost of building is complete so there is no additional cost.
How often are the users, power users, abusers going to access the API?
I am thinking either charge more for your service and include it and add rate limiting, authentication, 2FA, etc to add some friction.