Here's what we use at work: http://www.microcom.us/eoc3220.html $140 each (one at each location). We've gone up to a mile with not so much as a glitch, though they're rated to 10 miles. They require a modicum of line of sight, so you might have to put them on tripods on each house, but for a total investment of under $400, you really can't go wrong.