Pimp My Novel has an interesting (or, maybe, discouraging) post on the profits on book sales and who gets them. Note that discounting has an impact on royalties.
More recently, the same site did a post on sales and advances relating to children's books. In describing the difference between middle grade and YA fiction, he said, "MG plots tend to center on the protagonist's internal world, whereas YA plots are more complex and are more concerned with the protagonist's effect on his or her external world."
I had never heard that before. I will have to think about it.