The cost depends on many factors. Mainly on the complexity (including number of illustrations, style and size) and the intended usages of the illustrations. For example, the same illustrator will normally charge much higher for one illustration that will be used for advertising purposes worldwide without time limitation, than if the same illustration will be used in a single country by a low circulation magazine for one month.
So, in order to get an exact quote, you should send to the illustrator a detailed brief including:
1. How many illustrations will be in the same project and assigned at the same time.
2. Which will be the size and style of the illustrations.
3. The exact usages of the illustrations like picture-book, printed ads on a specific magazine, billboards, site’s template for resale, packaging etc.
4. Each usage duration.
5. Any geographical restrictions, like for usage in a signle country, specific number of countries, without geographical restrictions etc.
6. Which will have the right to use it. Describe if it is for personal use, by usage by one company only, a group of companies, for resale etc.
7. The cost of the rough drawings if agreed a separate cost for them.
8. When the illustrator will be paid in full.
9. Percentage of front-payment, if agreed so.
10. Rejection fee.
11. Possible rush fee, if for example you need the project to be completed by tomorrow.
If an illustrator knows all these information should be able to give you an exact quote. Please also notice that one illustrator is expected to price your project more or less than other illustrators will do, depending on his experience, skills, status and pricing policy.
If you are an illustrator, here are some additional information: Development Process of Cartoon Characters and Mascot Illustrations for Illustrators.