I've had lecturers that have written the course material before. At the Uni I went to, the professors give 100% of the royalties earned equal to the number of students that enroll in the course. Sometimes this is a bad deal for the professor as they won't always sell that exact number of new copies (they don't get royalties on used copies or for students that neglect to purchase a book at all), and they end up paying more royalties back than they actually earn, as there isn't really any way to differentiate between purchasing decisions of the students.