You're billed per token usage. GPT-3.5-Turbo price per 1K tokens is quite low now.

I kinda made my own Custom ChatGPT with Python (and LOTS of coding help from Web CharGPT). It evolved from a few lines shitty script to a version that uses Langchain and has access to custom tools, including custom data indexes and has a persistent memory.

What will ramp up the cost are things like how much context (memory) you want the chatbot to have. If you use something like a recursive summarizer, that summarizes a text by chunks over and over until the text is below a set length, that also uses many API calls that consume tokens. Also, if you want your chatbot to use custom info that you provided to it, solutions like LlamaIndex are easy to use, but require quite some tokens per query.

On my worst month, with lots of usage due to testing and without the latest price drop, I reached 70$.