TACAN400: You must have been reading the investment analysis fairy tales cooked up by the ADS-B program office and the vendors. When avionics were bought a few years ago for the Alaska Capstone operation in lots of 100 and 200 the cost of the avionics alone was more than your quoted total cost and then they needed to be installed. It cost the earth for the first time in a given aircraft mark/mod (how about 20K?) but once the design had been worked out and the STC issued it came down to 6-10K dollars per aircraft.
Nobody is doing away with primary radar and current VOR/DME if they already have it (they don't in parts of Alaska or Oz which makes ADS-B attractive). If it involves changing from a known and proved installed technology (like VOR/DME and primary radar and xponders) then it is a transition and they are HE!!.
Don't tell anyone but the picture on the scope with it's maps and data blocks and such is totally computer created and is not linked physically in any way to a primary radar. In fact, the only reason for the transponder (why did somebody name it secondary radar, it has nothing to do with radar) and the primary radar to have anything to do with each other is so the transponder ground antenna can spin around. If it does so pointing the same direction as the primary it makes it easier for the computer to produce the track file of beacon targets reinforced by primary targets and take the ghosts, angels, stationary, and other junk out and dump it on the floor.
The next cool radar thing is the integrated tracker that takes the output of multiple radars (primary or beacon) and puts them together and produces a data stream to make it easy to display a "radar" picture of any area of airspace you like without needing to know which facility the data came from or pulling your hair out to mosaic outputs from different sensors with custon software.
The inherent problem with ADS-B is the "D" part, it is dependent on the aircraft knowing where it is and telling everybody or somebody (ADS-A and ADS-C). Until that navigation is fairly bullet proof,as well as the communications, it will not likely be relied on in terminal or approach airspace. In oceanic or similar, sure, why not, it is the same functionally as a position report and there is no radar or ground transponder coverage out there anyway.