as far as i know there are two types, passive and active.
active - you have a key pad in your car, if you don't type a code in before you set off they ring you and ask you if the car should be moving. This has a monthly fee attached
Passive - when you find out your car has gone you ring them and they track it. No monthly subscription as they are not monitoring the car constantly.
I have no idea of actual costs though. You would have to get some quotes.