Of course they can put textbook work on their CVs, depending on the field and what’s customary. And no, you can’t generalize about what they make. Depending on the field, many pub houses are small and/or run on a shoestring. The have to cover costs first, before sharing profits. DH made maybe 2k (over some years)for an important research book in his specialty, the only English source for that material. He might make $100-150 or 1k for an entry in a larger book or reference set, depending on what it was for, how long and specialized, and what the market was. Plenty of work was free. Humanities.
To make a half million pounds, there has to be a broader market than the classroom or grad studies.