There are always so many debates going on about illustrators charging way too low or charging way too high. At the end of the day, you get what you pay for. If you want your book to sell, draw attention and enhance your story, you need a professional illustrator that has the necessary skills to do the job. It's not enough to just be able to draw a picture.
If you are looking for an illustrator or if you are a budding illustrator wondering what it takes or what you need to work on, focus on the following skills: