Writer's Fallout Shelter

I feel a little odd just waltzing back in here after being gone for so long. I didn't even follow this board for very long, so I feel like I'm new all over again. Based on the last few posts it seems the board has been pretty quiet though, so I hope nobody will mind if I try to get things rolling again with a bit of a question.

I was wondering what the general opinion is on illustrations in novels. Not page-by-page like you would see in a children's book, but maybe four or five full page illustrations scattered throughout the book. My main concern is how to find an artist, and once you've found that artist, how do you go about approaching them with the subject. Should you wait until the book is completely finished, or should you let them know while it's in the works? I've tried looking online to see what an average fee for illustrations would be as well, but I always end up on sites about children's books. Does anybody know anything I don't about this?