I Cut My LLM Bill 40x and Rewrote Nothing: A CTO's Migration Story

Here's the thing: i Cut My LLM Bill 40x and Rewrote Nothing: A CTO's Migration Story Six months ago my CFO slid a single line item across the table. OpenAI: $4,800 for the month. I'd like to say I was surprised, but I'd been watching the number climb for two quarters. What actually surprised me was how little it took to bring that number down to under $200 without anyone on my engineering team writing new code, without a single regression, and without telling my customers anything had changed. This is the story of how we did it, what we evaluated, what broke, and what I'd tell any other CTO walking into the same conversation with their finance lead. The Real Cost of Vendor Lock-In I've been a CTO long enough to recognize the pattern. You pick a vendor. The vendor becomes the default. Procurement assumes you're locked. Your engineers build abstractions around their quirks. Six months later nobody can tell you what it would actually cost to switch because the switching cost has become invisible. It's just "how we do things." OpenAI was that vendor for us. GPT-4o handled our summarization pipeline, our customer support copilot, and a few internal tools I'd hacked together on a Saturday. We were paying $2.50 per million input tokens and $10.00 per million output tokens. At our volume, those numbers add up faster than you'd think because the output side balloons in conversational workloads. Here's the arithmetic that should scare every CTO: at $10/M output, every million tokens of generated text costs a dime on the dollar. If your product generates a 1,000-token response for 100,000 users a day, that's 100 million tokens a day, which is $1,000 a day in output alone. That's $30,000 a month. Just for one feature. The 40x claim I keep seeing isn't marketing spin. DeepSeek V4 Flash charges $0.18/M input and $0.25/M output. Do that math against GPT-4o and the comparison is brutal. Multiply your current OpenAI output spend by 0.025 and you'll get the rough number you'd pay for