How to charge for a design

I was recently asked to work up a design for an element to hold
eyeglasses on one of those “necklaces” people hang their glasses
from. I’ve never actually sold a design to anyone-one that they
would totally own and be able to do with it what they please. Does
anyone have any thoughts on pricing? Thanks so much.

Ali Baird
alf jewelry & metals
www.alfmetals.com