It's sort of backwards when you're talking about municipalities competing for property tax revenue. Boulder city wants property tax revenue, so they allow development in specific places where they reap that. Therefore the city owns land that isn't land within the city so they can maximize property tax revenue, but still have a undeveloped belt. The surrounding counties aren't a big fan of this relationship, but can't do much about it.
The city of Boulder needs to pay property taxes on that land to the other counties, and that money comes from the local taxes that Boulder residents have to pay.