The custom edition was cheaper than the regular edition and this is the only reason the professor did this. The book didn't seem to have very many used copies being sold as it was an advanced class and people were more likely to keep the book than sell it.
Just providing context to how a custom edition can be cheaper if the professor does work.
I've also had many professors write their own book and give it for free in pdf or let you buy it for $20 at the university copy center already bound with the option of buying supplemental books that they thought were of good quality.
Maybe my engineering department was different but it seemed most of my professors tried to work to help the students not have to pay large amounts.
-The biggest issue is that custom editions segment the global secondary market for a given textbook into slivers of useless fragments. Instead of Campbell Biology, Regular Edition being bought and sold between Stanford students and students at every other college in the country (this significant supply drives the price down), Campbell Biology, Stanford Custom Edition is only ever going to be bought/sold by Stanford students. This hurts students in multiple ways: 1) the bookstore is the only place they can BUY the book from. Period. 2) the bookstore is the only place they can SELL the book too. Custom editions are a perfect storm for bookstores/publishers.
-Custom editions are cheaper than what the regular edition costs in terms of what the bookstore would charge for each, but if you compare prices on the regular edition online, there are usually significantly cheaper prices out there than what the custom is being sold for at the bookstore.
-In the rare case where a custom edition is being used, but a regular edition is acceptable - the bookstore presenting both options and telling you to pick one will almost never result in you considering that the regular edition would be cheaper online. By adding this extra noise, they artificially make custom books look more affordable.
I agree - there are tons of professors that are on the right side of this fight. Textbooks SHOULD be free. And the fact that custom editions are even adopted is proof that professors care about students. They're just a terribly poor solution which reinforces the problem instead of solving it.
Give the publishers credit - custom editions are a genius business move.