Depends on the API.

Generally I prefer flexible pricing, since we might need to scale up - i.e. if we double some limit, I want to be able to pay $60 rather than be blocked.

Some other considerations:

1. Have a free tier to let developers play with your API. i.e. first 1000 calls per month are free.

2. If you do variable pricing, let people set a limit and alert them before they hit that limit.