I know, BCrowell, I acknowledged that sometimes the old edition won't do. Students in some concentrations will be unluckier in this than others -- than most others, if they check.

If you're just looking for the same ISBN at a discount outlet, you're still getting robbed.

I hardly ever bought a new book at Harvard after the first year. There is the occasional book for which no earlier edition doesn't exist, and occasionally an earlier version won't do, but more often it will, and it was usually available in the used book stores around the Square. (That was a long time ago, but even if those stores are gone now, used on Amazon will give you an even better selection.)

In liberal arts courses particularly, you don't even need earlier editions by the same publisher; all "classic" texts in literature, history, psychology, etc. can be picked up dirt cheap. It's true, you'll have different editorial notes and glosses than the other students -- bring up these points in class participation, as if from your own vast side-reading on the subject, and completely cow the teaching assistants, sometimes even the professor. It's fun!