The fact that the author’s investment is less in a second edition than in the first doesn’t mean the book gets a free ride. A new edition still requires editorial and production attention from the publisher and has to undergo numerous stages, just like a new book.
The author has to go through all the old material, recognize where updates are needed, and incorporate new content, making appropriate deletions as needed.
The new edition has to be edited: checked throughout for consistency with the old material, making sure sense was not lost with deletions, making sure global changes (such as to terms) were made, making sure errors were not introduced. It’s donkey work and doesn’t pay much. Then it has to be proofread. And that really doesn’t pay much for intense and very specialized work that not just anybody can do.
Page composition has to be reworked where alterations have changed makeup. Textbook pages typically have a far more complex design and composition than a book of straight text, especially when color is added.
New illustrations may have to be searched (a whole job in itself, just finding pix that illustrate the text), and permissions obtained, usually for a fee that becomes part of the book’s cost, and/or new artwork commissioned. Tracking the permissions file and supplying copy for the list of credits is a whole job for somebody.
Index has to be redone—another specialized job for people with uncommon talents and endurance.
And of course the entire book manufacturing process is the same, only probably more costly than for the first edition just because it’s more recent.
“Regular” books don’t require extensive permissions and use fees, custom artwork and/or photos, indexes, fact-checking, high reliability of content, and a high degree of care for accuracy and freedom from typographic and production error. These days “regular” books may not even be edited by the publisher. The error level in content of ordinary books has gone way up in recent years. Textbooks still have to maintain standards.
You’re free to spend your money any way and anywhere you want. I just think asking other students to pay for your water bottle out of their tuition fees is a mistaken set of priorities.