It's not going to the authors. From Danielle Kurtzleben … How Your Textbook Dollars Are Divvied Up:
According to figures from the National Association of College Stores, an average of 21.6 cents of every dollar spent on a new textbook will go to the bookstore, whether for personnel costs, operations, or income. For Kadue's $289 [Calculus] textbook, that's around $62. Another cent of every dollar pays for the freight of shipping a heavy book around, so subtract another $3 from her cost. That leaves around $224 that goes to the publisher, or around 77.4 cents for every dollar.
NACS no longer receives information from publishers about where textbook money goes, but as recently as 2008, they provided that cost breakdown. At that time, around 15.4 cents of every dollar went toward marketing the textbooks, 11.7 cents went to the authors, and the largest chunk—32.2 cents—went to the basics: paper, printing, and paying publishers' employees.