As someone in the USA I've compared GPS to an Airtag and the difference hasn't shown to be different enough to bother with GPS. Considering how cheap an airtag is and the difference in battery life - it's a no brainer to use an airtag. The density of iOS devices in the USA is such that it's very comparable to GPS in my experience.

This is a cool project, though. One thing I didn't see is, how much does it cost to make this in practice?

I wish Androids had a way to talk to the Airtags :/ iphones have that cool proprietary technology that points you in the direction of the Airtag even if no other phones are nearby