Hello! Ish from Wolfram Partnerships here.
To use the Wolfram plugin for ChatGPT, you need to have a ChatGPT+ account (which is $20 a month, you are correct). No additional license is required.
OpenAI APIs do have a separate cost, which you can find on OpenAI's pricing page. I do not know if a ChatGPT+ user receives discounted OpenAI API pricing, that is a question best answered by their sales/support teams.
If you'd like to have a chat about plugin use and our experience with it, happy to have a conversation. Would you send us a note at partner-program@wolfram.com so someone on my team can reach out to you?
Thanks,
Ish