It's exacerbated by the fact that once a new edition comes out (which can happen every few years) the old versions have much less worth for new students. So you

It's exacerbated by the fact that once a new edition comes out (which can happen every few years) the old versions have much less worth for new students. So you could buy a $50 copy of an old version from an old student, but if it's different enough from the new version then it may be more hassle than its worth (page numbers not matching up, exercises being different etc.).