If models get smarter, why does the same task cost more?

Not a complaint about pricing. A real question I could not answer until I went looking.

The intuition is that a better model needs fewer attempts, so a job should get cheaper. My bill kept going the other way, and the price per token had gone down, not up.

The answer turned out to be in the docs. Reasoning models produce a block of internal work before they answer, and OpenAI's guide says it plainly: those tokens are "not visible via the API" but are "billed as output tokens". Anywhere from a few hundred to tens of thousands of them. Anthropic bills the same way.

So the answer you read is 200 tokens. The output you paid for is 12,000.

The part that changed how I work: a vague input to a weak model gave a wrong answer cheaply. A vague input to a strong model gives a right answer after the model spends real money working out what you meant. Ambiguity moved from an accuracy problem to a line on the invoice.

Curious whether anyone here has actually measured the reasoning token field across a prompt rewrite. I write about this at slimsnap.ai/blog/smarter-model-costs-more, and I build a screenshot tool, so treat my interest in the ambiguity part accordingly.