Well........your ? is hard to answer. I take it you are looking for a way to give a quote.
One needs to know what the size/shape/decorations/number of servings are.
Most designers figure final price on that information.
Example: You give this information
Cake is 3 tiers; coverd in fondant; 'simple' decorations.
Even that information is very limited.
Are the tiers round, sq, hex or other shaped?
Is it a special flavor or multi flavors? What kind of filling is used?
Describe 'simple'. That can mean even more/extra care must be taken as there are no decorations/flowers to help hide any imperfection that might develope. Many times, even the most beautifully decorated cake hides *something* - some little flaw 
All of the above is discussed with the bride and then a per serving price is figured depending on her answers/information.
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