Hi,
Yes the plugin is free (and there is a pro version for more features), but openAI has a cost.
All pricing is available here: https://openai.com/pricing
It’s not easy to say exactly how much the cost of the API can be, because openAI counts it with “tokens”. But as far as I know, the cost of “gpt-3.5-turbo-1106” (recommended model for comments) is very, very cheap. I’ve done hundreds of tests over several days, and my spending for each day is never more than a few cents, for a total of less than $1 for February.
The cost of gpt-4-1106-preview is more expensive than GPT 3.5, but it is still quite cheap and a bit more accurate.