[cepth]

It's in the linked article:

> (A Great Courses lecturer earns a royalty that varies according to how highly viewers rate his performance; the base royalty is 4 percent of the course’s gross revenue, but that rate can rise to 6 percent if a course receives high enough evaluations. The average royalty is about $25,000 a year for a course.)

The Great Courses likes to say that they have a very low "acceptance rate". Maybe a hundred professors are considered for each course that actually makes it to market.

The royalty for older courses will also naturally decay once the content goes "stale".