Let me answer that with a statement and an anecdote.

The statement: The proportion of costs I outlined, from my point of view, have always been true. I finished my first degree in 1997.

The anecdote: All the professors in my computer science program (I'm not going to name-drop, but I had some great teachers that insisted that I learn scheme, then Java, then C, then assembly) either had no required text or gave syllabi that referenced several versions of their listed texts. The context was that we were free to get the best deal possible on the texts.