You can run a local copy against cloud SQL with go run bqcost.go credentials.go --cloudSQLProxy=true
You can also run a local copy using SQLite, but I need to figure out a way to make this work without breaking deploys to App Engine Flexible.
Known Issues
Downloading the state of all the tables is incredibly slow. It needs to be parallelized.
The "get table data from BigQuery" job is just a goroutine. If the instance restarts, the job is stuck forever and you will never be able to access the project. This should be split into smaller chunks that get retried.