Here's how I imagine it came about:
Publicity Intern: Man, you couldn't pay me to attend a Trump book signing.
Publicist: Mervyn, by God, you're a genius! (Publicist gets on the horn with Trump's people.) Okay, I just thought of something great. Let's pay people to come to the book signing!
And so it is, my friends. A new era in what we abashedly call the book business.