A professional illustrator knows prepress, production, professional color scanning, layout and design... in addition to illustration. The publishing house offers a similar contract to illustrators and authors, which is usually an advance on royalties plus a royalty contract. Depending on the complexity of the illustrations, you should expect to pay between $75-$125 per page in the Southern US. $75 per page for simplistic cartoon or line drawings, and $125 for realistic people and extra expenses for the illustrator such as travel or hiring models.
If you are interested in my illustration for your book, I only consider self published books with full payment up front. The reason is most self published books don't sell enough to make a royalty contract attractive. I can show you samples of my work in different media and advise you throughout the production process. The first step is to review the manuscript. All manuscripts are kept confidential.