If you only need one, and you have a place to plug it in, the $25 GeoHopper is the best place to start, I think. You can’t buy just one Estimote, you have to buy a pack of three.
For your particular use case, another option is the Securifi key fob, which costs $15. It’s not automatic detection, but for arming and disarming an alarm mode, it’s a good solution for many people.