Passport data largely from https://www.passportindex.org with fallback to Wikipedia scraper
Monetary per diem estimates come from a multitude of sources and then blended together into a interpolated raster which the backend uses to query so that there are no empty spots for cities with no data.
Transit connection estimates are based on GTFS and airline sources
I also expose some of this data in my other tools:
- Reverse flight search https://unli.xyz/flights/
- Gap year planner https://unli.xyz/city/calc/