This tool itself is FOSS, but it is designed to work using your own OpenAI api API key, so that is a cost that goes to OpenAI.
However I did add an OPENAI_API_BASE_URL env var override option, which allows you to point to any LLM endpoints that conform to the OpenAI spec. I use this at work because we have a proxy service called Credal which has a drop-in replacement API that the company pays for (they handle company IP and security stuff for us).
I am not an expert but I believe there is a tool our there that provides OpenAI compatible endpoints for open source LLMs, so in theory you can use this tool without any charges at all. I haven't played around with that myself though.