That's not to say there there might not be developed websites - using 3rd level domains - that might sell or have sold at different times due to the quirk-du-jour of various search engines favoring sub-domains or the link love given to a sub-domain website.
I suspect the value isn't "in the domain", only in the domain as matters now stand for that domain as an active site.
Sometimes such value can be fleeting. On the other hand there are folks that reportedly make loads of money based upon their awareness of such fleeting opportunties for great traffic love affairs with search engines.