I run a fleet and we have NAVMAN trackers ther mint, can see ever move they make and speed, locations etc. we pay 80p per vehicle per day which i think is about the rate if you hire 1. you can buy them out right but not sure how much they are, ours are on 3 year contracts but if you were to seel your car in this time all they do is swap it to the next vehicle.

In your own private car though id say there not really worth the money, unless you live in some crime hotspot and there going missing every night? maybe then think about it.