A cost site where the schema itself refuses to store a figure without a source.
Home-service cost guides are almost entirely invented. Numbers get copied between sites until nobody can trace where any of them came from.
Costs are derived from federal producer price and wage statistics, and the database enforces a citation on every published figure. Where no defensible source exists, the site says so rather than filling the gap.
A cost guide that can show its working for every number it publishes.