One of the toughest challenges for professional designers and illustrators is accurately estimating the cost of illustration. It is also important to consider other elements associated with a project. Illustration costs can vary a lot from market to market and even from one client to another. A wide variety of art styles, techniques, and work processes also make quoting flat or hourly rates challenging.
It's also important to remember you're not just purchasing an illustration. When you purchase an illustration for business use, you're also purchasing the rights to reproduce it in various formats and media. The more use you plan on getting out of your illustration, the more the illustration is worth. Consequently, prices for pieces of art that will be used a great deal and in many settings may be more expensive than, say, purchasing something to hang on your wall at home.