I read another post oddly similar earlier today that has more explicit data on that authors codebase: https://codepointer.substack.com/p/cutting-llm-token-costs-w...

TLDR; ~3-4% savings to actual API costs with rtk, caveman, and headroom combined, but nothing tangible on if those cost reductions came at a cost of quality. By their calculations, rtk saved them $4.96 on a $926 bill.