Even the first Power Stone is up there. I thought I would do myself a service of getting just the jewel case and then pick up the loose disc later for cheap. Except that hasn't quite worked out as I planned...
While some might think that disc only are worthless and nothing like carts only for the presentation
Times have chanced and disc only for the better games sell for a nice amount sometimes even 50% or more also for the more pricy games.
Worthless games disc only are worthless but than again that is expected.
I'm not suprised since disc only is first of all really hard to find, usually it is at least boxed. discs without boxes will probably damage unless you would put it in another case.
Furthermore you also have buyers who need replacement discs and yea, a box and manual without a game has allot less value and obviously not playable. I'm pretty sure that especially these groups of people keep the price for disc only games high.
Also a minor group who collect disc only because it is cheap(er) but that's a small group.
Good luck man finding a disc only will be a tough challenge if you want to get one for cheap.
On a positive note since value of disc only games are worth a nice amount you will find more lose discs on ebay of the better games since it is worth the effort.