The thin ones like for maths and stuff are yeah about $25, but the thicker ones for lets say legal studies, which have all the moduels in them are about $30.
Also when you look at the HSC make sure they are worth buying, eg, if you do modern history the excel book doesent even half half the topics of the course in it only the popular ones, so the moral of the story is look before you buy...
EDIT: and in some HSC ones, you can a CD with it, i think it has past papers on it or something, not sure havent look to hard at it, and they go for a bit over $40.