Another expensive album not mentioned here is Galaxy Odyssey. suruga-ya's buy price, i.e. what they'll pay you for the album, is 40,000 yen as of today. Though 4 people on vgmdb own the album, which suggests it's not ridiculously rare, further highlighting the disconnect between "expensive" and "rare."
In fact, a decent way to identify the most expensive albums is to sort suruga-ya's buy list by price. Here is a link, which shows Donkey Kong Country 2 to be the most expensive soundtrack by far:
http://www.suruga-ya.jp/search_buy?c...83%BC%E3%83%A0
As an example of a rare soundtrack which isn't expensive: there is a pressing of Final Fantasy Symphonic Suite (PSCR-5184) of which I'm the only owner on vgmdb:
http://vgmdb.net/album/39107
So the album is probably rare. But because it's a reprint - meaning the music itself isn't rare at all - the album likely wouldn't sell for very much.