Google has what I would call a generous free tier, even including Gemini 2.5 Pro (https://ai.google.dev/gemini-api/docs/rate-limits). Just get an API key from AiStudio. Also very easy to just make a switch in your agent so that if you hit up against a rate limit for one model, re-request the query with the next model. With Pro/Flash/Flash-Lite and their previews, you've got 2500+ free requests per day.