I bought the $200 plan so after my extras started routinely exceeding that. Harsh.

But, let me suggest that you stop thinking about planning and design as "prompts". I work with it to figure out what I want to do and have it write a spec.md. Then I work with it to figure out the implementation strategy and have it write implementation.md. Then I tell it I am going to give those docs to a new instance and ask it to write all the context it will need with instructions about the files and have it write handoff.md.

By giving up on the paradigm of prompts, I turned my focus to the application and that has been very productive for me.

Good luck.