I Accidentally Spent $400 on GPT-4o in One Month. Here's How to Never Do That.

Last year I deployed a GPT-4o powered support chatbot for a small SaaS. Traffic was modest — maybe 500 active users. I checked the OpenAI bill at the end of the month.

$400.

The app was supposed to cost $40/month. I'd done the mental math: "1,000 API calls a day, maybe 500 tokens each, GPT-4o is $10/M output... that's like $50/month, fine."

What I forgot: my system prompt was 600 tokens. It was sent on every single call. At 8,000 daily calls (users were chatty), that's 4.8M extra input tokens per day just from the system prompt. At $2.50/M, that's $12/day — $360/month — from a prompt I copy-pasted and never measured.

That's when I built the LLM API Cost Calculator — a free tool that covers 18 models, 7 currencies, a live token counter, and a Compare tab that ranks every model by cost for your exact workload. Let me show you how to use it properly, and walk through the math so you never have a surprise bill month.