I'm going to toot my own horn a bit here and mention the
Sangokushi Taisen 3 OST. It was only available with the most expensive version of the first edition Sangokushi Taisen 3 arcade DX pack (binder/deck case/etc) and was only for sale through Sega Direct. Damn good OST too!
No idea if it's actually worth anything, but I'd say it qualifies as at least "hard to find".