I am the general manager of a "casual-dining" restaurant. I won't say which one, but think in terms of TGIFriday's or Ruby Tuesday. Anyway, we could not survive if we nickel and dimed every customer that walks in there. I've read people talking about the charge covering the cost of washing the plate, extra garnish, etc. Those costs are so minimal that it doesn't make sense in my estimation to charge $2-3 for the extra plate.
Guests in my restaurant constantly ask for extra BBQ, extra pickles, extra crackers, etc. We give it to them at no charge. Let's figure this out: A gallon of BBQ sauce costs me about 8.00. There are roughly 16 cups in a gallon at 8 oz. per cup. If I give a guest an extra 2 oz of BBQ sauce, it costs me about about .12. I would estimate that in labor, hot water, and chemicals you add less than a nickel to wash the ramekin that it is served in.
I'll take the $30-50 that the average family of four spends in my restaurant over worrying about that .12. If I charge, say, a quarter for every extra side to cover the cost, then that family is going to end up going to one of the dozen or so other restaurants up and down the street that don't charge for that.
Again, I'll take the sales. This may sound crude and uneducated, but I can say with some degree of knowledge, that the higher your gross sales, the easier it is to make your budgets. Period. SO, anything I can do to bring in that $30-50, I'll do. Even if that includes not charging for an extra plate.
Just my 12 cents ...