[DuctTapeAI]

Depends on your business model, how useful/unique the API is and many other things.

I would suggest making it free to get an API key and run a small number of queries (or a low rate limit). That allows devs to build against your API without needing to take out their credit card. Beyond that, I'd suggest something like $1 per thousand calls and add a bulk discount if your larger users are providing extra value to you.