It was already mentioned but I'll second the original
Sonic Adventure OST. It rarely shows up on auction sites and when it does, it usually starts in the $200-250 range.
Yakuza 1 & 2 OST, same deal. Only one on YJA right now and it's $244. Sigh.
Keys of the city ("Leading Company" jazz arrange CD).. never seen it show up for sale anywhere and I've been watching for it for almost a year now.
Gungage Original Soundtrack, same deal as Keys of the city, I've never seen it show up on any auction site and I check regularly.
Elemental Gimmick Gear Original Soundtrack - yet another one that never shows up anywhere, ever.
Seems a lot of the CDs I want are super hard to find and/or expensive.