They make alot of this kind of thing for training field dogs.
Tracker makes a radio system for $499 that is good for up to 12 miles.
Innotek does a similar one that is good for about 7 miles.
$599 with two transmitters that work on 4 different channels.
Garmin makes a GPS unit that is a transmitter and hand held GPS unit/ reciever. $599 for the combo and $199 for the transmitter if you already have a GPS unit that would work.