Recently a co-worker's home was burglarized. I use the term "burglarized" not robbed, because technically a robbery only occurs if the owner is present, and forced to turn over possessions. This misfortune, however got me to re-evaluate my own security.
According to ADT, "Homes with ADT® monitored security systems are three times less likely to be broken into than those homes without a security system." This does not surprise me, because the average burglar will not go near a home with an ADT yard sign, or window decals. The interesting part is that the signs and window decals are the cheapest part of owning a security system.
A yard sign and decals can be purchased on ebay for around $30-$50. These identifiers obviously don't provide the protection of an actual system, but it can greatly affect the odds of a break in. There are even companies that specialize in making fake security signs, but I heard burglars can tell the difference.
So, if you don't currently have a functioning security system at home, consider getting some authentic signs. It could be the best $30 you ever spent.