Pricing sign painting work is something of a dark art, and a lot will depend on the nature of the client and your level of experience. Assuming you're not working for free (check here to see if you should) then you need some way of estimating what to charge.
"If I'm working for free, I'm working for me."
First, ask if the client already has a budget, or can get one together. You should add that this will help you understand the scale of the job, and propose something suitable for the resources they have available. However, this can often result in a stand-off.