Doing a wedding cake in May with 12", 10", 8", and 6" French vanilla with alternating vanilla BC filling and chocolate ganache filling. Bride now wants to add a 14" square dummy to it. How much extra should I charge for dummy cake?

I am in a cake club, and I was in a discussion one day with some fellow members about this. Some charged for dummies by size and one charged by serving. After hearing their discussion, I thought the serving charge made a lot of sense. She happened to charge 1.00 per serving on a dummy. She is very very good by the way and people didn't complain about this. Although your not baking anything decorating is just as extensive. That being said, I don't know how much you charge normally, you may want to adjust that up or down!!

I don't think that making a dummy should be cheaper. Styrofoam is expensive and it is just as difficult to use. The pain of trying to get it back is just not worth it to me. Maybe charge them full price with a small refund for returning it?

I would charge her $0.75 to $1.00 less per 'serving' for the dummy cake. So if you cahrged her, say $4.00 per serving, it would look like this:

6" (18 servings)
8" (32 servings)
10" (50 servings)
12" (72 servings)
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= 172 servings @ $4.00

+14" (98 servings)
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= +98 servings @ 3.00 to $3.25

Dummy cakes are still a whole lot of work. Be careful that you don't burn yourself here. If you are going to add dummies, make it worth your time, and make sure your price reflects that. By doing a dummy, you are only saving nominal ingredient costs (maybe $15 for that 14" cake?) but you still have just as much decorating/setup time. And you need to pay for the cake dummy. Don't short-change yourself.
HTH