Back-of-envelope: what happens to my AI bill if 1,000 people sign up at once

Hi PH, Noor here, solo founder of Ember (emberai.site). Ember runs on Groq's gpt-oss-120b. My first rate-limit errors (HTTP 429) didn't come from users. They came from running my own test suite. So before I market anything, I did the math. Everything below is my estimate, not measured on production.

The numbers: Groq lists this model at about $0.15 per million input tokens and $0.60 per million output tokens (from third-party pages; I'm verifying in my console). A first roast uses roughly 6,000 tokens. A signup that also generates a plan is roughly 15,000. A check-in makes 5 sequential model calls, about 8,000 to 10,000 tokens. That works out to about half a cent per full signup, and roughly 10 cents a month for a free user who checks in daily.

Where it actually breaks: not on money, on speed limits. I'm on Groq's free plan today: about 30 requests and 8,000 tokens a minute, which is less than one signup's worth every two minutes. The paid developer tier is reported at around 250,000 to 300,000 tokens a minute, about 17 signups a minute. So 1,000 people signing up in the same hour would need about 15 million tokens, roughly an hour of queueing.

What I'm doing about it: upgrade before marketing, set a spending cap, make the model name swappable (it's copied into 5 files right now), and plan a second provider. The same model is listed cheaper on other hosts, so a backup could also save money.

For anyone who has launched an AI app: when a spike hit a rate-limited provider, did you queue, fall back to another model, or hard-cap new users?