Prices won't come down because most of what you've written is bollux.
I worked in the printing industry for a while. The lowest cost part of the publication is the printing of the book itself, assuming you don't have an idiot for a printer. The costs are in the distribution marketing, and payments to all the middlemen involved in producing the book. Granted it's been a while since I was at dear old State, but our textbooks were black and white, and just as outrageously priced. Started in Astro, so I spent as much if not more on books than the Lit types who had more numerous volumes. The reason given of course was the expense of the type fonts to set all those damned equations. Believable before the advent of computer typesetting, but not anymore. At the time I was spending about $450/semester for books, and that was more than 20 years ago.
If anything, the used book market would drive prices down, if profs weren't in cahoots with the publishers. War and Peace hasn't changed any since Tolstoy wrote it. Newtonian physics has changed less since he published his work, and all the translation for that has been done long since. Which means small changes don't necessitate changing the text for the course. So if the publisher puts out volume XIII, volumes X, XI, and XII should still be good. Oh, and if the publishers wrote good usable reference books, there wouldn't BE a secondary market because students would keep them for future reference. I still have my physics books and kept a fair number of the math ones as well.
As to the last, yes the authors are a major cost, but good books don't necessarily take a long time to write. They may take a long time to edit because most profs can't speak let alone write a simple sentence, but that can be worked around. Another problem might be illegible long-hand notes that some poor schmuck at the printing house has to decrypt into some semblance of English. (My astrophysics prof had a sheaf of hand-written notes that were run off at the local copy shop every year, completely indecipherable.) No the problem is most printing houses aren't interested in signing contracts with new authors because despite the economies of scale, academic books lose money.
I'll also note that I did a back of the envelop calculation when they quoted the 30 day rental figure and went, 'Ah, so that'll be at best 10% less than if they bought the book to get them through the 90 day semester, which is easily compensated by selling your books back.'