How I Cut My Claude API Bill from $340 to $67/Month (Free Interactive Workbook)

My Claude API bill hit $340 last month. Not because I was doing anything wrong - I was just building something that scales, and the costs were scaling with it. I spent a few weeks auditing every single API call. What I found was embarrassingly fixable: about 75% of my costs came from three patterns I kept repeating without realizing it. The 3 patterns eating my budget 1. Sending the same context on every call I had a ~2,000 token system prompt going out with every request. No caching. Pure waste. Fix: prompt caching. Two lines of code. Cut 40% of the bill overnight. 2.