Personal opinion on why Disco I & II's and RRC's are cheap. They both have the same heritage, a chassis and drivetrain derived from the Defender.
All use the same engine, the 215 Buick V8 that while light and all aluminum, was early to the manufacturing processes for aluminum engines. Those older technologies could not manage porosity and heating issues. Even Buick had issues with engine manufacturing yield, forcing moves to cast iron. Discoveries experienced those issues as head gaskets and slipped liners. Then mix in owners propensity to defer maintenance and you have a recipe for high failure rates and expensive repairs. There also use to be a disparity between the cost of American vehicle repairs and European vehicle repairs with the latter being much higher as mechanics would be fewer and higher cost. That is no longer true. Just recently someone said "Nice Rover, bet that cost a lot to maintain". When I told them it was the old Buick V8 and easy to work on they gave me this confused look highlighting they didn't have a clue.
In addition the Defender/Discovery/RRC was a full frame, solid axle design living in a world that moved to independent suspension and more cup holders than anyone needs. The heritage of the design took its toll on used prices.
Then add into the mix quality issues that Land Rover didn't learn until they were owned by Ford, then BMW, then Tata. The same path as Jaguar. I think the XJ6 is a great car but it is also "cheap" on today's market. Oh, and Lucas the Prince of Darkness reputation. Well earned in the 60's/70's but not really a issue later on. One of the common Rover "electrical" issues are the window rocker switches sourced from ......... Mercedes Benz. LOL