I have been researching for a few days now.

I believe the JayCar tracker is only 2G which will become obsolete in a couple of months.

I have also researched DIY but the problem is the cost of a cheap sim card plan.

I have narrowed it down to two companies that sell the hardware for around $350 - $400 (including the first 12 months monitoring fees). The second year of monitoring fees are around $10 per device per month which is cheaper than a cheap sim card plan. This cost is almost recovered with a discount on my car insurance. One of he devices will immobilize the vehicle. I have researched the statistics and although it is rare for a late model vehicle to be stolen without keys, it is the first thing thieves look for when doing a house break in. In fact a work mate had his car stolen in that exact fashion (keys found in a house break in and car stolen from driveway).

When my Ducati 748 was stolen it was locked up outside of my work (in full view of a window). A van pulled up and they lifted the bike in the back all within a minute. It was never recovered and police later told me it was the sixth bike stolen in the area that day. The insurance after that was a nightmare. The forfeited the remaining balance of the premium, I had to pay the excess and they made up their own market price of the bike (not taking into account new tyres and a recent $1000 service). The insurance company refused to insure my new bike if I was leaving it parked in the same suburb (where i worked). If i had had a GPS attached to the bike I would have been alerted (via a Geofence) the bike was moving and I could have alerted the police immediately (or given chase myself). At the very least I could have told the police where the bike was at any given moment.

I can see the unscrupulous side of a GPS tracking device but I think the day will come when these become standard in vehicles (as they are in phones today).