I'd reckon on paying about £300 for the box, then chucking a few quid a year on a pay-as-you sim card to keep it running. This is based on market research we did last summer. a
Typical price point for sat tracking is about £1 per day for 'live tracking', which you don't need.
Our boffins are building an ARM9-based linux box which will compete in this arena, but much more programmable. One obvious feature to program in is a text message / voice call when the vehicle moves outside programmable parameters - e.g. radio tag not in range (on drivers key fob, not at normal hours, outside geofence etc.
Tracker is a very very good solution for theft, particularly the monitored version. However, you can't get the data yourself as it's a closed system. Your own system on the roof is more flexible, but probably won't work out any cheaper.