peter c is correct. If your tracker uses TCP connection, it connects then disconnects every time it sends an update which is rounded up to 1p, very expensive!!!. You can avoid this and use a UDP type tracker such as the AVL900 which keeps the connection open constantly. This will cost about 50p per month for a vehicle travelling a couple of hours a day.
You can buy the AVL900 tracker at trackit247.co.uk
Hope this helps.