Now we're just haggling over the price
If I tried to buy a Harvard co-author, for example, I would expect nothing other than horrified looks. There's the reputational cost if word leaks out, of course. But on top of that, a cash-for-co-authorship deal is so weird that even a cash-hungry professor would be very skittish.Actually, it is not quite as weird as Bryan suggests. In the textbook market, one often finds a book with a seemingly odd pair of coauthors: Introductory Textbook by famous professor X and unknown professor Y.
Typically in such arrangements, Y does most of the hard work of writing and revising. X is recruited, perhaps by Y's publisher, because his stature will bring attention to the project. Sometimes X demands more than half of the royalties, even though he does less than half of the work. This just goes to show that marketing can be more important than production.