Start at $180 all the way up to $999. The cheaper they are the less warning you get, about 2 seconds from my last cheapy. Pretty useless really. A heap of false alarms to. I would reccomend buying a Bel. You will need to spend at least $400 tho to get any useful sort of warning and minimul false alarms. They work in the same way as the sensor eye in your tv and vcr that picks up your remote. The cheaper Detectors have a internal oscilator that can be picked up by the police radar detector detectors (VG2). They still claim to be undetectable, because when they sense they are being detected they cloak themselves by switching of the internal oscilator. This doesn`t work all of the time as I have experienced( and had my detector taken and been fined for the illegal use of a police detection device). The more expensive Bel brand radar detecters don't have and internal oscilator at all, therefore are totally undetectable.