That's actually a very important part of eruv construction. In order to build an eruv, you need permission from the city government. Every 20 years or so, the Rabbi in charge of a city's eruv will meet with the mayor and have a ceremonial "rental" of the property encompassed by the eruv, and will pay the city a dollar.

Eruvs often make use of existing walls, utility poles and cables which are on public property, but if any extra poles or wires are needed they are paid for entirely from donations.