That's reasonable—glad to know you considered it. Two alternatives to the free tier for single or small-batch uses so that you don't need to deal with payment information and reduce sign-up friction:

User completes a human-level task (for another API even) somewhere between CAPTCHA and MTurk to earn a few API calls.

Do something with ads near or even on the result to earn a few API calls. (Prototyping dev eyeballs are valuable.)