Luxembourg has insane tax revenue per capita because of its status as an international tax haven. A program that might be hardly noticeable on Luxembourg's budget could put a big dent into the budget of an American city.
It's very possible it's the same in Iowa City.
“The truth about Zohran’s free busses” by Breaking Points:
https://www.youtube.com/watch?v=P275SobdE-s
clickbait headline (of course) but gives a lot of facts about the proposal and talks about other places they’ve tried it.