I don't think it's super rare; there are usually 1-2 copies floating around Yahoo at all times, and they sell too,
sometimes. Seems like it's 15 to 25,000¥ at the moment.
I'd say it's a combination of it being only available on Konamistyle (only with the limited bundle, not even as a
separate item!) and the fact that the game and the music are good... better than the previous episodes (OK, that's kind of personal).
No idea, but maybe 5,000 copies, or even more? I have the feeling many people have it, most Castlevania fans and collectors certainly do. 50 people have it according to VGMdb, which is a very large amount of people. Crazy rarities/obscure albums are usually owned by much fewer people, as far as this site is concerned at least.