If you add wireless smoke/heat detectors, plan to spend about $1,300 for DIY alarm systems. That’s about half of what a professional alarm company would charge. Monitoring service, which notifies the police or fire department, can cost $40 or more per month. But if you shop around, you can get it for about $10 per month. Some alarm installers recommend eliminating window transmitters and installing motion sensors instead. These sensors are similar to the motion detectors that turn on outdoor floodlights. That approach can cut costs, but we don’t recommend it. Motion sensors cause the majority of false alarms. Worse, they detect burglars only after they’ve entered your home. That can create a dangerous situation where you come face to face with a burglar in your hallway. Door and window transmitters are the only way to detect break ins immediately.