There are a few things clients do not realise about illustration. And that it, even if a client asks an illustrator to produce an illustration for them, they still do NOT own the copyright.
The copyright int eh UK always stayes with the illustrator unless he or she sells it to the client for a fee over and above the cost of the actual illustration. This then ensures the client has exclusivity on the illustration and the creator cannot resell or re-use it.
What happens in reality is that the illustrator sells the client a license at a fee that reflects the usage and complexity of the illustration. For example, and illustration that is done for a private function that will only be shown to, say, 50 people, will be charged a lot less than an illustration due to be printed on 1000 tee shirts, and that will be charged a lot less than in illustration for a large multinational bank that is going to be on all marketing collateral, online and offline, thoughout the entire UK.
These usage details need to be taken into account before the illustration can be quoted on. For more information, see the following 2 books I have recently read and gotten all my information from