I'm working with someone who wants me to design a mural and have someones else paint it. I've never done something like that before, and they seem extremely frugal. I know they're being paid several thousand dollars for this mural and offered to pay me only 100$ for the design ...

How should I go about explaining that isn't an acceptable value, especially when they basically will be using the design to make a lot a large profit.

Is this even common?