I occasionally work in the textbook industry. In my experience, what you're paying for varies from book to book. This one series I've worked on over the last decade has had many editions, and we put about a year's worth of work into each one, often having a crunch period at the end. So I know that in some cases at least, one edition can be quite different from the next, and much of that effort can be in the form of making sure the most up-to-date information in the field is included. I can't speak for everyone though; I'm sure there are some out there that only put in a token effort.
As for the pricing of the books, that's out of everyone's hands but the publisher, and I've never heard an author be happy that the price of his book went up. One of the most commonly cited reasons for price increases (by the publisher anyway) is the used book buybacks (probably sounds familiar to gamers). Publishers get no revenue from used sales, so they raise prices, and then everyone tries everything they can to avoid buying new, and the whole thing escalates. It's gotten bad enough that a while ago, I searched for a book I worked on and one of the first results was an eBook on a torrent site. Although I have to admit, I felt a bit of satisfaction that something I had worked on was in enough demand to be pirated.
I agree textbooks are overpriced, and that in many fields it is possibly an outdated medium that needs to make the same kind of shift that music did. Some publishers are already switching to eBook format. That said, it is well-documented that reading a physical page and reading something on a screen have major differences in terms of comprehension. My English professors would often remind us to print our essays to proof them rather than reading them on the screen, and they seemed to be right; I found far more errors on paper than on the screen. So, it's possible that in some fields there will always be a place for physical books. But we have the same thing in gaming; some people prefer the disc.
I'm not sure, beyond the buyback programs, what leads to the inflated prices; you'll have to inquire with the publishers about that. It certainly is not perfectly correlated with quality; the most expensive book I've bought for my classes at my current school was short, black and white, and filled with bad and outdated advice. I'm not just saying that as a student, but as a TA; I later TA'ed a lower level class that also uses that book, and we changed an exercise to be "first do it the way the book says, then do it right, and explain the changes you made." Don't point the fingers at the faculty or university; it was the students of a previous class that voted to adopt this particular book.
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