A lot of the cost is regulatory. I used to work at a mobile provider, and it took months to get permission from all the various government agencies before we could actually start building. Even if the tower is in BFE, you still have to get all your plots to the FCC, you need EPA signoff for batteries and fuel tanks and such. Plus there's always state and local permits of various kinds. We had a custom workflow application just to track all of that and there were dozens of steps.

Cell towers aren't very expensive on an ongoing basis, but every few years you're rolling out the next big technology (we went from analog to 1x to 3g to LTE while I was there) and it's a headache.