i have seen this so many times...
i'm thinking it's time we replaced api keys.
some type of real time crypto payment maybe?
Oh please no. And the "alternatives" to API keys aren't going to help much either, they'll just add friction to getting started (as reference: see the pain involved in writing a script that hits gmail or calendar API)
Prepaid only is a fantastic idea, especially for dumb-ass startups. Limiting your liability to $100 or so sound like a big-ass W.