A.S.
Our home already had an ADT system in place. We pay about $40/month. Keep in mind that you have to have a land line with a security system. I think you can get varying levels of security; of course, you'll also pay more. For example, you can determine if you want the alarm to activate when the window of your doors are broken versus any window on the basement or first level. Even if someone came through the window, the motion detectors in your house would turn on the alarm.
Alarms are a good deterrent, but we were told by a police officer that some burglars just move quickly through a home and know that they have X amount of time before the police are called by the security company. Be consistent about setting the alarm, too! It's very easy to fall into a pattern of not using it, particularly when you're coming home with kids and arms full of groceries, etc.