Originally published at API Cost Lab .

Last updated: October 2026

Almost every developer hits the same moment: the first real OpenAI invoice. You expected tens of dollars. You got hundreds. And your first instinct — that something must be broken — is usually wrong. Nothing is broken. Your bill is high for predictable, fixable reasons, and they're the same reasons for nearly everyone.

This is a diagnostic checklist. Work through it in order, and you'll find where your money is going.

The Usual Culprits

1. Every call goes to the flagship model

This is the number one cause of bill shock, by a wide margin. Somewhere in your code there's a model name, and it's the biggest one — because that's the default, that's what the quickstart used, and nobody revisited it.

The problem: not every task needs the flagship. Classification, extraction, formatting, simple Q&A, summarization — these run fine on smaller, cheaper models.…