You can install one yourself for less money.
My security system cost about $20, a series of little battey-powered alarms on all the windows and doors. For $300 you can get a pretty darn nice system that will call the service for a monthly fee. If you can use a hand drill, consider doing some shopping and following the instructions.
ADT is for old people who just want it done immediately and can pay.
Get a no solicitor sign. Those people are usually casing neighborhoods for easy marks and looking in your living room to see what you have inside.
There are other measures as well. Trim bushes, install yard lighting and/or motion detectors, put signs in trouble spots (my father had a warning:you are being recorded sign BEHIND his garage, which would have been a staging area for burglars given the layout. Neighbors and passers wouldn't see the sign, only trespassers).
You can find a home security for dummies book for $10 and get some great ideas. Or just go to borders and read it over a few cups of coffee.
Some people put police union stickers on their window, not sure if this helps, but it would make me think twice.