An alarm system is worth every penny if it prevents a burglary or saves your family from an intruder. It's just hard to place a value on things like motion detectors in advance since you could spend hundreds of dollars a year on something that works only when it's triggered by the family dog. You must assess your risk and exposure and do what's best for your home.
You'll get some discount on your homeowner's insurance if you have a detection system. How much will depend on your insurance company and the system. It'll also vary with your neighborhood and its crime statistics. There usually won't be much difference you add internal motion detectors, although you may feel safer.
Types of Detectors
Alarm systems range in cost and complexity. Some people feel safe with a simple device that turns on a security light when it detects the slightest motion. Others sign up for a system that will send reports to an outside monitor when it picks up some out-of-the ordinary motion. How much you spend will depend on the system you choose. You'll get the most benefit from interior motion detectors when you live in a big house with several exterior doors.
A typical monitored alarm system has detectors on all of the doors and windows and a few more inside spots. They'll sound a siren in the home and alert a monitor through a telephone when it picks up movement. Most companies provide the equipment for free, but do charge you to have it installed and maintained. In 2012, an installation fee cost $100 or so, and a monitoring contract about $50 a month. You can usually keep the basic system and drop the monitoring contract after two or three years.
Do It Yourself
You can install your own motion detectors anywhere in the house where there's an electrical fixture or outlet. In 2012, these typically cost $50 or less, but you may have to pay an electrician to run wiring if you don't have an existing fixture you can replace. If that's too much for you, there are wireless sensors that can plug into any electrical outlet and monitor motion in the area.
The size of your house will play a major role in your motion detector decision. If you're in a 1,500 square foot home where you can see every door and window from one central point, you won't need much internal motion monitoring. But, if you're in a rambling ranch or two-story, you'll want a motion detector in hallways or connections between the house's wings. The cost will depend on how many internal monitors you install and whether you sign up for off-site 24-hour monitoring.