Some companies sell trackers with an initial low sales price but tie you into an expensivce monthly contract.
When you purchase a new mobile phone you usually purchase at a discounted rate but you are tied into a contract that can last for 12-24 months. This is how the phone company redeems its initial loss.
Most GPS Tracker companies do likewise and tie you into a contract so beware of this before you make your purchase. Always find out what the monthly fees are and if you are tied to a contract.