> I wonder how difficult it would be to build an app that lists and estimates fares for different local car services.

Google Maps (on Android, at least, but I assume generally, I just haven't checked elsewhere) has that feature already, though when I've checked what it showed it only listed Uber. Even if it is currently Uber-only, the feature's UI makes very clear that its intended to be a ride service time & cost estimate comparison feature, so...