The cost depends on many factors. Mainly on the intended usages, but also on complexity (including number of illustrations, style and size). For example, the same illustrator will normally charge much higher for one illustration that will be used worldwide without any limitation, than if the same illustration will be used for specific advertising usages like in a single country, for six months, only on billboards.
So, in order to get an exact quote, you should ask your potential client for a detailed brief including:
1. The exact usages of the illustrations:
– if they need a buyout (all rights) or specific usages like packaging, printed ads on specific magazines, billboards, TV commercials etc.
– If they need exclusive rights and for how long.
– Any Geographical restrictions, like for usage in a signle country, specific number of countries, without geographical restrictions etc
2. Each usage’s duration.
3. How many illustrations will be in the same project and assigned at the same time.
4. The size and style (in case you work in more than one styles) of the illustrations.
5. Which will have the right to use it, like usage by one company only, a group of companies, rights for resale etc.
6. The cost of the rough drawings if agreed a separate cost for them.
7. Information about your front payment, full payment.
8. Rejection fee. In case they stop the project after you have started working on it or finished it too.
9. Their deadline. When they want you to deliver the rough drawings, the final project and also set a rush fee, if for example you need the project to be completed in extremely tight deadlines.
As soon as they will provide you all these information, you will have a better understanding of their needs and you will be able to give them an exact quote depending on your 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.