The R-520A is the rarest of the military Trans-Oceanics, so that's why it fetches so much money. The R-520 is an earlier military model that is not quite as rare, but is still very collectable. If you want a TO from the tube era, I would recommend that you look for a nice H-500. There's always plenty of them to be found on eBay. If you want a later solid state radio, then look for a D7000Y which dates from the early 1970's. I have a small collection of these radios and the D7000Y is definately my favorite to use!
Check this site for tons of info on the R-520/URR:
www.thisoldradio.com