You can ship an LLM feature in an afternoon. Figuring out what it costs to run usually happens later, when the invoice shows up and someone asks why. A few minutes of token math up front avoids most of that.
Here is how the pricing works and how to estimate it.
Tokens, not words
Providers bill per token, not per word or per request. A token is about 4 characters of English, so "Hello world" is roughly 3 tokens and 750 words lands near 1,000 tokens. Input and output are billed separately, and output is almost always the pricier side.
GPT-4o is $2.50 per million input tokens and $10.00 per million output tokens. That 4x gap is the part people underestimate once responses get long.
