GPS trackers, mains power or battery, low cost?

Has anyone got experience with GPS trackers? Can be wired in or battery powered, sim card, discrete?

Most of the units we've seen are proprietary and expensive or designed to just have one or two units to monitor. We need 300+ which also means management will be tricky. Our units are to recover stolen property, not for day to day tracking but they'd still be expected to "ping" at least once a day.