How is the pay for the author of a University textbook?

Hello,

I realize authors obtain a royalty out of each book sold, but my question within a question is how many copies are sold on average (per year) for a good quality University textbook in say, Engineering? Does anyone know how many copies typically sell, and how much the royalties are for some publishers? I can't find any lists or references on the subject.How is the pay for the author of a University textbook?
Most university textbooks are written by university professors that have to publish or perish. Their main income is their teaching job. They probably only make a few thousand for the textbook and probably only a few thousand of the textbooks are sold.