It's too bad that DME/DME RNAV isn't more widely available. The only real reason it's so expensive is that there isn't much demand for it since GPS (usually) works fine. Electronics-wise, it's not much more complicated than a transponder. Unlike a GPS, it does have to transmit, so it will always be somewhat more expensive than GPS.
The other problem is that there's a limit to how many aircraft a DME station can serve at a time (about 100), but I believe that could be greatly expanded if aircraft weren't pinging the DME so often. A position fix every second is generally fine, and it could be even more infrequent if you have a cheap inertial system to fuse with it that can fill in the track for a few seconds between pings.
