Oblivion GOTY only came with Knights of the Nine and Shivering Isles it was missing horse armour and the other thing
It didn't include http://www.uesp.net/wiki/Oblivion:Plug-ins, honestly.
Of course, the general methods of DLC and GotY editions are a bit different now than they were then. (See: FO3 and FONV, with all DLC, etc.)
edit: not sure what the guy above me is talking about.

Anyway, I'd guess that the Fallout GOTY's dropped in price reasonably quickly because 1. game's already been out for awhile; 2. most of the people really hyped for the DLC likely had bought them already. GotY editions are targeted at bargain hunters and latecomers, so it's not a surprise they get discounted. (in my case, I'm not that interested in those DLC. I've got two for FO3, none for FONV. Haven't bothered with either GOTY. In Oblivion's case, I got the GotY long after it'd been out, because I got tired of not using mods that said "requires SI", and the GotY was cheaper than an SI disc.)