The CDs are rare because After Forever is, sadly, one of the less known bands.
Look at the prices on Amazon.com:

Invisible Circles (Limited Edition) [IMPORT] (Audio CD) - 28.99
Decipher [Sweden Bonus Tracks] [EXTRA TRACKS] [IMPORT] (Audio CD) - 51.99
Prison of Desire [IMPORT] (Audio CD) - 31.49
Exordium (Bonus Dvd) (Audio CD) - 18.99

All prices are in USD. Exordium seems less expensive but that is because Exordium is something between an album and an LP: it has six songs, one of which is the one you just heard.