1. New editions are not churned out every six months.
2. In most cases you can get away with not having the latest edition for a class . Schools don't change the syllabus just because a new edition of a text book has just rolled out.
No, new editions are churned out every 12 months or so; of course it depends on the course. I remember my strategic management book was in its 14th edition, and its first editions was only in the 90s.
And the section numbers, case studies and questions get shuffled around too, so the previous edition is not sufficient.
The market is inefficient because the folks who choose which books to buy - the people who set the course - are not the people who have to pony up the costs. So it's in the interest of the publishers to maximize their gouging of students, and then spending a good fraction of the surplus on "incentives" for course setters to keep them on board and recommending the latest edition. The publisher who gouges the most has the most money to spend capturing these interests, creating a feedback loop, so for any given course vertical, there's a tendency to a monopoly.
Some schools are fighting back against this. For example, I know that the UCLA math department has a policy that professors are required to search for a replacement textbook once a textbook goes beyond the third edition, unless there is a significant addition of material (e.g. entirely new chapters needed as a result of recent developments). The idea being: "If the author hasn't got it by the third edition, they'll never get it."
I believe this eliminated the majority of calculus/linear algebra/diff-eq texts from introductory courses, so it has become easier to find used copies.
Based on my experience in school you're right about the first point (though they are still churned out very frequently) but incorrect about the second point (at least at the school I go to- I suspect it varies from school to school). Professors frequently require the latest edition of a book. Sometimes you can use the old book, but oftentimes it makes it very difficult to do the homework as the chapter/problem numbers get scrambled around in every edition and new problems can be added in the new editions.
I can't really say since I don't have the 7th edition. In general though, they usually add another chapter or two to incorporate recent developments in the field. These are usually interesting but almost never actually used in class.
The only reason for the price difference (in used copies at least) is that all classes require the new one now, so there is no demand for the old one.
Forgive my ignorance, not having been to a US college - what happens if you just buy the old edition and (say) copy the pages from the new one in the library, or make some handwritten notes? I'm assuming they don't do a loyalty check on the contents of your book bag when you attend class. Do the book problems vary from edition to edition, limiting your ability to do assignments, for example?
Don't mean to be snarky but I get the impression that the relationship between US textbook publishers and academia is really the worst of all worlds, and seems like a case of the free market gone wrong, with few colleges working to offer a competitive (ie cheaper) option to students who don't want to pay ridiculous prices for books.
Mind you, it's great for autodidacts like me who can pick up slightly older textbooks for less than the cost of printing. Now if only Spivak would do a new edition of his calculus book...
