A boost mobile phone ($50), a data cable (~$8), $6 every month or so, and load the mologogo software and bam cheap gps tracker.
You will probably want to hack a nextel car charger into the cars electrical system somewhere for a more pernament solution. car chargers are $1 at (gasp) dollar stores.
A charge of a motorola i415 will go for a good 14 hours uploading points, im imagining you could put one of these phones in a box with a strong magnet and wip it onto the bottom of a car you wanted to spontaneously track too. Its like being james bond.