the biggest work ahead is contacting the site managers and drafting up a lease. you can pretty much forget any Federal site, you will get more homeland security red tape, and as a private individual, they will tell you flat out NO.
State and local governments usually are only slightly warmer- more so to entities they have working relationships with, i.e. CERT teams, RACES/ARES, and even then there is a process to go through. Again, as a private individual you will have a tough sell. They will be concerned about: 1)your presence there 2)interference to their operations, 3)-liability (do you have professional liability insurance? and of course, what benefit does it do for the site owner to have you co-locate? If you aren't paying rent, you have to be prepared to bring something to the table. Free tower sites are few and far between.
This is just the beginning. ASSuming you are granted permission IN WRITING (very important I might add), you can only do what is allowed in your lease. The agency may want you to use their approved tower crews to install antenna/feedlines, may ask for site surveys by their appointed RF engineers to ensure no RFI will occur, and then there is the cost of the antenna, feedline, lightning protection, power supply system, etc.
You should expect to budget a minimum of $5000 on the low end, $10K on the high end, to put your repeater on a professionally managed mountaintop site. This is of course, doing it right, and not cutting corners or bootlegging. This number could go up or down, we'd need to know more specifics like: the type of tower(s), available space, what tenants are there, what equipment is currently in use, the HAAT of the site, what your desired coverage is, etc to give a more accurate number. A good repeater antenna can run $2000 alone, good feedline isn't cheap either. Depending on who you are co-locating with, additional filters may be needed to ensure you don't interfere with someone else, and they don't interfere with you. This can be expensive.
As you can see, infrastructure is not cheap, and the most important part of the chain. Anyone can buy a repeater/duplexer and stick it up. Having it work well is another ball game in and of itself.
