The invoice arrived on a Tuesday morning. $340. For a side project I'd built in a weekend. A small LLM-powered summarization tool — users paste text, model returns a summary. I'd done the math before launching: roughly $0.002 per request, ~500 requests/day, around $30/month. Totally fine. What I hadn't accounted for: system_prompt_tokens = 800 requests_per_day = 2000 # not 500 — it went viral in a group chat input_price_per_1M = 2.50 # GPT-4o daily_cost = (800 * 2000 / 1_000_000)...