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GPS tracking is the easy part, transmitting that location to an outside party is the hard part, well the part that costs mo' money, because that has to be done via cellphone frequencies, i.e. 3g, 4g , lte, you can't count on free WiFi for vehicle tracking, lol.
So each vehicle needs a cellphone transmitter connected to it's GPS, and a unique ID so you know what vehicle it is.
And it isn't free to connect to local cell service providers cell towers, they are in business just like you.
You can read this article:
http://gizmodo.com/5691724/how-to-track-your-vehicle-on-the-cheap
And do the math for the monthly cost per vehicle.
GPS Vehicle tracking providers get bulk data rates and the GPS/transmitters(hardware) used in the vehicles at a bulk rate as well.
They also usually provide "recorded tracking", so you can log into your account and see where a vehicle traveled on any given day or night, lol, sitting at the Bar for 4 hours.
So it is really a matter of deciding what you need, what is the purpose for the tracking and can it save or make you money.