Occasionally a potential author will ask about compensation before they have even written a book proposal. These educators (typically) have often asked a loved one for advice and then come back to me and say, "My husband/wife/friend/relative says I shouldn't write a book unless it makes sense financially. Can you tell me how much I can expect to make?"
I understand the concern, I really do, but until I have a proposal in hand, until I can really "see" what the book will be and where in the market it will best fit, I can't really talk about compensation.
These folks still deserve an answer, of course, and in a nutshell, this is what I tell them. (Keep in mind that I'm referring to authors of textbook publishers for allied health. Nursing textbook authors often make considerably more because there are so many more nursing students than those in any single discipline in allied health.)
You're thinking about writing a book for, say, medical assistants. The MA market is pretty solid, well over 150,000 students a year, but unless you're planning to author a book for a core course that all MA programs offer, you're looking at only a slice of that MA pie.
Even then, books for a core course are competing against a host of other textbooks, so take that slice and then divide it again.
If you're planning to write a book for a less ubiquitous course, or perhaps a book that supplements other books, your slice of the pie will be really small.
So know going into this that your book will most likely not make you rich, it just won't. You won't be able to send your kids through college on the royalties, but twice a year, in April and October, a nice little paycheck will arrive in your bank account.
If you're lucky, you could purchase some nice patio furniture with it or a lovely new rug for the dining room.
If you're really lucky, if your book sells really well, you could put a hefty down payment on a new car or install a new deck off your back door.
Only a precious few authors, though, make six-figure incomes for any single edition of their book.
Textbook authors by and large write because they have something to say, because they want to make an impact on their profession, because they want to earn the praise and respect of their peers, or for any number of other wonderful and fulfilling reasons, but the seldom do it for the money.
If someone's main reason for writing a textbook is to make money, they'll probably be disappointed. I'm not saying they won't make money, because they probably will. I mean, why would I sign a book that I believe isn't going to make us or the author money?
So a bit of advice if you're considering authoring a textbook. Don't do it purely for the money; you won't succeed. Do it for your profession and know that in April and October you'll receive a lovely little gift, which will make you glad you decided to write.