How much do bestselling physics authors earn from their books?

[Nano-Passion]

I've read that authors of books get paid very little and have many challenges in getting their book published.

But my question is, how much would Michio Kaku make out of "Physics of the Impossible" or Stephen Hawking out of "The universe in a Nutshell"? Both of which where bestsellers and popular.

 

[Vanadium 50]

It's a buck or two per book.