There's four main albums for the TV series, plus a remix album and a "best of" sort of dealy that also contains three exclusive tracks, and then there's the soundtrack for the movie and also a maxi-single which contains the remaining songs from the movie.
Those few CDs cost so much because they're imports, and they're genuine non-bootleg copies from Otaku.com.
There's also a five-disc box set which contains many of the songs from the separate albums, but also a whole slew of exclusive tracks, but that thing's out of print. I doubt I'd be able to find a non-bootleg copy, so I'll likely just download all the exclusive tracks.