Tower space is always a premium.
Sometimes, depending on who you approach, how nice/accommodating they are, and their tower situation, you may, like the other person stated, rent the space for hundreds to thousands of dollars.
In some cases, especially if you approach with the repeater being for emergency communications, you may only need to pay for power for your repeater setup, cut the grass around the tower site (and guy wire stations if it is not a self-supporting tower), or you may get it all free.
Where I am, we have various situations. My club's VHF repeater sits on a television tower, well over 1000ft in the air (two antennas, one for RX and another for TX, with either at something like 1300ft and 1600ft, I don't remember the exact numbers). At that site, we only have to keep the grass and plants cut back around the control building, tower base, and the guy-wire stations. Very small price to pay for one of the best repeater setups in the piedmont triad area of NC.
Another is a UHF repeater. It is in a bad tower position as the antenna is blocked by the tower on one side, so it limits coverage, but it does work fine if you're in the capable zones and is on a government emergency services tower and at no cost.
There are various other tower and building locations available that may be provided free of charge or at cost of power.