To the OP,
Do you mean a tracking device you can buy off fleeBay, wire in yourself, stick a SIM card in and SMS it when your feeling lonely and have it tell you where it is - OR - do you mean a Thatcham Cat 5/6/7 Tracker solution that is monitored remotely by a control centre and you pay an annual licencing fee for? You call them when you notice your P&J is missing, or they call you, depending on the solution you choose.
If your looking for a cheap and cheerful tracker DIY install, then fleeBay is your friend, and any of the GPS trackers from £14 upwards will work, as long as you wire in and locate the device correctly. Add a PAYG SIM card, and your sorted. Price generally increases depending on how complex you want to make the install (everything from remote stop to interior cameras over 4G network)
If you mean the Thatcham approved device, then you can purchase direct from their website at an all inclusive price including installation. The annual licence fee is EXTRA, so don't forget to figure that into your budget.
You can find Tracker products on fleeBay at much better prices that the Tracker website, including installation - genuine products, installed by independent car alarm specialists, and to a high standard. You pay them for the product and install. They usually come to you at home/workplace to install and set up the whole solution including testing with the Tracker control centre. You then activate your chosen licence plan over the phone and pay your money.
I know some people "don't" want their cars back, and I fully understand why, but if having a device fitted gets some of these thieving scumbags apprehended and picked up by the police then Im all for it. It may not save your car, but it might save your mates, or another member of this forum from their activities.