Actually the radios you would use on the roving GPS units are only about $50. That’s the XBee SX (900 MHz OEM RF Module | Digi XBee SX 900 | Digi International). Those can only transmit at only 20 mW, but a roving unit never needs to transmit, only receive. The SX Pro is 1 watt and that’s what I use for the base station. Those are $300 (Actually Digikey.ca is selling it for only $130 right now). Line of sight, it can reach 10-20km or more. They come with different firmwares, such as mesh. I reflash them to the 9Xtend hopping firmware using the XTCU software.
The only thing about these modules is they use the worst form factor ever. They are designed to be surface soldered to a parent board. Horrible design. For our purposes a socket is far superior, so you can easily replace the module if it fries, which is something I am wont to do sometimes. Unfortunately you can’t even buy the special spring headers that could make a socket.