Generally - a well designed (and implemented) system will use proper filtering and isolation, and won't have a problem playing nicely with other nearby transmitters. The farther away (in frequency) the two (three, or four, +) transmitters are, the less chance they'll cause any appreciable amount of intermod. There are intermod calculators available online - including one thru Radio Mobile. Plug in your frequencies, and they'll run the theoretical results. Any decent site manager will require that YOU mitigate any interference or intermod problems that your transmitter(s) create. Most operate on a "first in" basis - meaning the guy who was there first doesn't need to fix his system (assuming it was properly built), you need to fix yours so that it doesn't interfere with any other existing user. Once you're in and operating without causing problems, the onus shifts to the next guy coming in. If the site is owned by a company specializing in Vertical Real Estate, you're going to want to be sitting down before you get the price quote. A smaller company might be more willing to talk, but you're going to need to make a pretty strong case for what you plan to bring to the table that's a benefit to them (rather than just a benefit to you.)