So I thought this was interesting and looked into it.

The website has a reference for where they get their big mac prices:

> Big Mac Index measures the average price of a Big Mac at McDonald's in the United States and is famously used in the Economist's Big Mac Index to measure inflation.

And a link to the Economist article (https://www.economist.com/big-mac-index) where they further link to their methodology: https://github.com/TheEconomist/big-mac-data

The USA average seemed high to me, and I found this resource: https://www.zippia.com/advice/how-much-big-mac-costs-states/ - Nov 2021 - which suggests $4.40. But the Economists data/csv file really does say $5.81 for the USA... so that's where I bow out!