Check out simplisafe.com. I *highly* recommend. When we went to Disney in December, I couldn't bear the thought of leaving a house full of Christmas gifts next to our oddball renter next door neighbors. Told DH there was no way I was leaving without an alarm on the house.
We have a decent sized house (3,200 sf) so I had to buy a few motion detectors and sensors for all 4 doors on the main floor, as well as the basement windows. It was about $200--much cheaper than ADT.
The monitoring is only $14.99 month and they do a GREAT job. I've set if off on accident a few times (oops!) because I'm still getting used to the fact that we have an alarm, lol. Every time, they are on the phone with me in less than 30 seconds.