As far as I can tell, it seems like it is free for right now. This is part of the ChatGPT plus service, so that kind of makes sense. Most consumers don’t want to have to worry about paying for API calls, they want a more simplified billing. Perhaps they might offer x number of plugins/integrations for a specific tier of ChatGPT? Or they might rate limit how much you can use the plugins, like they do with the GPT-4 model and ChatGPT.
They may charge the people making the plugins, but I haven’t seen any mention of that so far. I think it is more likely that plugins with simply be a bonus feature that paying customers get access to, in order to entice more people to sign up for a paid version of ChatGPT (or higher tier paid version of ChatGPT).