It actually looks like you're missing one of the biggest -- and now, according to the Supreme Court, legal -- new loopholes for cheap textbooks.
One of the most important changes in the textbook market that every buyer should be aware of: last year in Kirtsaeng v. John Wiley & Sons, Inc. [http://www.supremecourt.gov/opinions/12pdf/11-697_d1o2.pdf] the US Supreme Court upheld the ability for US-based students to buy copies of textbooks printed in foreign countries (often at huge cut-rate prices) [see also Ars Technica: http://arstechnica.com/tech-policy/2013/03/thai-student-protected-by-first-sale-supreme-court-rules/]. This means that searching online bookstores in India, Indonesia, Pakistan, etc. will often find the EXACT same textbooks (usually with slightly different ISBNs, and slightly cheaper paper) for HUGE discounts in the 60-95% range.
To stick with your math example: I recently bought an international edition of Walter Rudin's Principles of Mathematical Analysis (Amazon $121 http://amzn.to/1U74CD7) for $5 (and it even happened to ship from within the US for $3). Not only was this 96% off of the cover price, but it was 78% off of Amazon's rental price! How amazing is it to spend almost as much to purchase a book as it is to ship it to yourself!? I'll also note here that the first edition of this book appeared in 1964 and this very popular third edition is from 1976, so it isn't an example of "edition creep", but it's still got a tremendous mark up in relation to other common analysis texts [http://amzn.to/1V3QVlm] which list on Amazon for $35-50.
Hint: Abe Books (a subsidiary of Amazon http://affiliates.abebooks.com/c/133691/77416/2029?u=http%3A%2F%2Fwww.abebooks.com%2Fbooks%2FTextbooks%2Finternational-editions.shtml) is better than most at finding/sourcing international editions of textbooks.
The same Calculus textbook you mentioned as a Sixth Edition can be easily found in paperback for $55.44 (I only did one search to find it, but I'm sure a little elbow grease might cut the price further) and ships within the US for less than $10. And because many students are apt to take the multi-variable calculus course as a follow up, this textbook also includes that material as an added bonus as well. http://www.abebooks.com/servlet/BookDetailsPL?bi=14224082117&searchurl=sts%3Dt%26sortby%3D20%26kn%3D%22International+Edition%22%26an%3DDeborah+Hughes-Hallett
The other option you leave out is purchasing the fifth edition, which isn't substantially different from the 6th edition and which can be easily found for less than $6 in good condition and including shipping.
Of course all this belies the true discussion of the how's and why's for why the textbook market is overpriced in the first place. For some of those ideas and suggestions of how we can fix them, I've written a short essay: "To Purchase, Rent, or Pirate? The Broken Economics of Textbooks in the Digital Age" http://boffosocko.com/2015/08/24/to-purchase-rent-or-pirate-the-broken-economics-of-textbooks-in-the-digital-age/