It depends on where you live, what you spend on ingredients, how long it takes to make it multiplied by what you think your hourly wage should be, how much you will charge for part of your utilities and supplies (cake boards, box, electricity, water, etc.), sales tax if you have to charge it, and other costs. There is no one set price of one cake.
Completely agree with the above...i find i have completely under charge myself when i underestimate the time scale of a cake. So now i break down each part, ie covering and decorating the board, leveling, filling icing each tier, decorating each tier, stacking the cake, final touches - Into hour or half hourly slots in a realistic time frame, and add this to materials & expenditure costs. Don't know if this helps 
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