The most up to date LLM is $0.0200/1K tokens (I believe this is tokens returned, approximately 1 word = 1 token, but not quite). I set a billing limit of $50 a month and spent a couple hours playing around this afternoon. It has cost less than $1 so far.
If you go to https://beta.openai.com/account/usage
you will see things like:
text-davinci, 5 requests
187 prompt + 218 completion = 405 tokens