This is a good question Jay, I'd like know how to price work myself. The old "what the market will bear" , is hard to employ for me since I don't know where to start. If anyone out there has done this sort of thing with success I'd be very interested in knowing too!
Maybe a percent of his profits if he is just starting out. If you are just starting to design for others, maybe a lower charge to get your own name out there. Assumes, of course, that you are not already famous!
When I was starting out in photography I found the best thing to do was ring up others who were already operating and pretend you were an interested client and ask them how much they would charge. Some people have pricing on their websites too. Once you've done enough research just think where you would comfortably place yourself amongst these, considering your level of skill and experience. Also consider how long each one takes you. If the amount you are thinking of charging is broken down into a per hour amount, is that an amount you are happy with?