A publishing executive whom I met at a reception years ago said that the only millionaire professor whom he knew became wealthy as a result of writing textbooks was Richard Hofstadter, the government and political science scholar.
As for the textbook budget at our house, Lake Jr. did fine with older editions, used books, and the relatively cheap Global Editions. We spent a fraction of the list price for Lake Jr.'s STEM textbooks. He just purchased the Global Edition of a Probability & Statistics textbook which the campus bookstore is selling new for $150. A fresh copy of the Global Edition sells for $35. Same content and same build quality as any typical American paperback book.