Illustrator/author?

answers1:There are published formulas for what you ask. No guesses
are required. The Graphic Artists Guild publishes the "Pricing and
Ethical Guidelines Handbook." It not only gives what you should
charge but also pro forma (standard) contracts. It is an excellent
resource and well worth the purchase. I use it for my book cover
illustrations, magazine illustrations, and other art licenses that I
have granted for the use of my art.
answers2:I write books and illustrate them too, so I need not share
with anyone. I think the author gets the royalties on a book, and pays
the illustrator in advance usually.