Our Fieldnet pivot system uses off-the-shelf 900 Mhz radios to communicate for several miles (the base station can go about 6 or 7 miles). I think they use normal XTend 900 Mhz modules, 1 watt.
Another possible solution is a simple app on a cell phone that communicates via the internet with your base station, and talks over serial or bluetooth to the RTK receiver. A bit more complex though.
I have a couple of Trimble integrated autosteer units, currently using just WAAS. I know that the steering computer actually talks over RS232 to the receiver, but it uses a proprietary trimble protocol that is partially documented. But I was thinking that an arduino could intercept data from the receiver, decoding it just enough to find the frames with GPS position data in it, and then substitute data from the RTK receiver, and then pass it on to the nav computer. The nav computer would still think it's using WAAS, but it would be fed a very precise, repeatable RTK signal.
Basically I'd love it if I could use a cheap RTK receiver to provide navigation data to existing autosteer solutions that I already have, but unfortunately everyone is all proprietary and locked up so that they can cash my checks each year.