There's an opensource rtklib which supports a handful of inexpensive receivers: http://gpspp.sakura.ne.jp/rtklib/rtklib.htm
You also need either a lot of storage locally or a fast data link between the base station and the rover - phones actually have both of these but they cost money
So, $0.50 instead of $0.01? :)
I used to have an updating map on the web of all the GLONASS satellites the Russian government had put into orbit, but it was taken down.
Right now, Kinze are heavily touting their autonomous system for grain carts: http://www.youtube.com/watch?v=MhA5aIw7xNk
John Deere and CNH have similar systems in actual production, but still require an operator to be present for safety reasons. And safety seems to be the big limiting factor. While the technology is basically there, nobody wants to remove the operator just yet. Though in this video, someone disabled the safety sensors to go fully autonomous: http://www.youtube.com/watch?v=zU4liQvrcm4
Most farmers are pulling their RTK data over wireless/mobile internet connections, rather than having their own base station. I don't know of anyone who maintains their own station. A few companies have placed base stations around various places in the countryside and will happily sell you their data.
In Europe (and of course in Germany) the federal governments offer realtime gps correction data via IP or 3G (radio was shut down a few years ago).
They offer a precision of 1cm-2cm used together with GPS/GLONASS
PDF: http://www.sapos.de/pdf/Flyer/2004Flyer_e.pdf
(Sorry, German language only:)
We successfully navigated a car down a twisting road, about 10' wide, without looking out the window, so it was a useful mechanism. It became less important, of course, once the US Government removed the S/A error from the GPS signal and the commercial use of GPS took off.
In fact I just found a wikipedia article all about it - http://en.wikipedia.org/wiki/GPS_augmentation :)