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...
