You ship your AI feature. It works. A week later your OpenAI bill is $400 and you have no idea which of your users caused which $0.05.
This is the single most underrated metric in production LLM apps — cost per end-user — and it's surprisingly easy to instrument if you know what to do.
Here are the three approaches I've found work in practice, ranked by setup time.
Approach 1: Wrap your provider client (5 minutes)
Works for Express, Next.js Route Handlers, Fastify — anything that has a single OpenAI or Anthropic client instance.
