I got a unit here that I bought when my motorcycle was stolen. I am yet to fit it, but plan to do so. For a $100 I dont think you can go wrong. However, my problem is I am yet to buy a SIM card for it. You need telstra to get the coverage, but the cost is a problem. Once I figure it all out I will buy a second unit for my boat. They have 2 methods of tracking, one is real time via data connection, second is you send the device an SMS and it responds with its location.
Thinking a prepaid sim at this point, and just using the SMS method of tracking. Cost you bugger all till its flogged, then you smash it with SMS's to find it. Might cost you a bit when stolen, but who really cares?