Depends, I took economics modules at University, and that was the only subject where the mandatory course textbook was co-written by the module leader. Well, co-written is strong: they added ten exercise questions and some call out boxes to the default Pearson textbook. Naturally, it was 40% more expensive.

School and professor dependent. I had a few professors at UVA who authored the book being used in class.

I also had a few who made copies of the chapters they wanted from whatever books and had the library hold those copies for us. We would just check them out normally, make our own copy (or read, take notes) and return for the next person.

Is it more likely? The most overpriced books I was required to buy were the ones the professor wrote.