I use claude code with an API key and pay per token, and the /cost command is very helpful.

And before people ask, it's because I have a very low usage and it's cheaper to pay per token. I'll have the odd month at $30, then nothing for a few months

It exists on my work enterprise account but not my personal account which is a monthly flat rate. I assume if I exceed my quota and I choose pay as I go then it will become available.