Send me a PM if you wish. I have some insight into this. Both me and SDDuuuude have a long history in this area. I recently helped out a guy that I surfed with in a similiar situation on a NorCal site. My wife use to be responsible for the site itself… so its a small world.
SDDuuuude was right about all of those factors but if I see some specifics it might clue me in to the sites worth. Usually unless there is something specific attribute about the site there are other candidates so you have to be careful in negotiations or they may select another site.
And just so you know my creditials… I spent over 15 years designing and optimizing cell sites for Sprint, T-Mobile, and AT&T. I have done some work for VZW as well.
Lastly, while most contracts are only for seven years, or so, I have seen almost no towers decomissioned in my career. In 95 percent of all cases I would say that it is guarnteed income for the next 20 to 30 years. In addition if other carriers join onto the site, which in many cases they will, there could be more revenue sharing.