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We typically don't put up our own towers if there's reasonable rent already on a butte/mountain. A lot of times if it's on gov't/BLM land, it's very difficult to put up a new tower if there's already towers and space available. It can take years to get approval.
We've worked with American Tower on a few sites, they're OK to work with once you know how their system works and you've done one tower deal. You'll use their mapping system to pick out a site, and then apply for it, figure out what you want to put on it and negotiate costs... and then before you hang gear you have to pay an engineering fee, usually around $2k. Depending on how full the tower is, what the surrounding pop is, and where on the tower you want to be... it can cost a lot or a little. They do have a WISP program that does give some better pricing, but they put you down lower on the tower and I think if anyone else wants to pay more and they run out of space, you get bumped or you have to pay more. The rates we've been getting are around $125-$150 per mo per antenna. If you change anything on the site you typically have to jump through a bunch of hoops and sometimes pay an engineering fee... so you want to make sure that everything that you'd possibly want is on the tower from the get go.
If you think AT is bad... I've heard Crown Castle is worse to deal with and more expensive... but maybe they're coming around to doing more business with WISP's.
There's also smaller regional tower owners that you should be able to find... they're typically the easiest to work with and their rates are often very good. The majority of our leases are with smaller regional tower owners. We'll typically visit a site with towers and there's usually some identification on gates or the fence, radio of FCC ID, etc. and then we'll just start searching in Google and making calls until we find someone that knows the tower owner.