There is a separate, optional feature called Supermaven Chat which is basically a wrapper around GPT-4o / Claude 3.5 where you can chat about fragments of your code, refactor them etc. This costs money, but you can also supply your own API key for OpenAI/Anthropic models. Or just use the web interface of any LLM if you just want to chat about your codebase.
It appears that for the $10/month you just get access to additional features (e.g. bigger context) + a budget of $5/month of credits. The credits possibly translate 1:1 to the usage costs of the underlying models you chose.
I asked it to add some missing documentation with the Claude 3.5 Sonnet model to a medium-sized Python file (2k lines) as a first test and that used up 13 cents of the credits. If it were working in a really productive way (where I'd also include more files as context), I'd probably burn through $20-50/day.
I'm wondering why they are not publicly showing some more transparent pricing. Yeah, this method will bump their signup numbers for investors, but it also absolutely wrecks their churn numbers, when people immediately cancel in the first hour.
It also has a pay as you go chat which is integrated into the IDE w/ hotkeys, however the above subscription gives you $5 free credits for it a month. This doesn't call their own model but stuff like gpt-4o which is presumably why they don't offer unlimited free usage.