I took a wedding cake order for a cake similiar to the one in the picture except the lady wants four tier instead of three
and also she wants a sweet treat table with 150 cupcakes
please give me an idea on what to charge?

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Depends on what size the tiers are, is it BC or Fondant, are there a lot of details?

how many servings does she need in the cake? A 4 tier can serve 125 or it can serve up to 200 or more. How many guests is she inviting compared to how many guests is she expecting to show up? Is she planning cake for 150 AND cupcakes for 150 and does that mean she expects 150 guests to eat two servings of cake? OR is she planning for 300 guests to choose between cake and cupcakes and if so, how will she determine who gets wedding cake and who gets cupcakes?

While some of these questions do not pertain directly to "how much should I charge?", they are questions that I would ask a client to help determine exactly how much cake they REALLY need, which leads into the conversation of "how much do I charge?" since it determines how much cake is needed.

http://www.costhelper.com/cost/wedding/cake.html

http://www.earlenescakes.com/business09.htm

Hope this helps

Of course the important question here is what should you charge per serving , not what we think you should charge or even what we would charge. This will take a little time and effort on your part.
First decide whether or not you actually want to make a profit, after you've made that decision calculate your cost per serving.
This is something you have to do, we can't tell you how much it costs you to make a cake.
Remember to take into account time, electricity, insurance, licensing fees, utilities, etc. etc. etc.
Once you know that you will know who much it cost you to make 1 serving of cake, if it comes out to .50 per serving and you decide you want to make $3 per serving then you'll know to charge at least $3.50 per serving.
Now the magic happens...take what you charge per serving and multiply it by the number of servings in a 4 tier cake, same with the cupcakes.

she is expecting 300 people originally she wanted six tiers and cupcakes but i did convince her to downsize the cake.
i don't really know the purpose for the cupcakes, I just trying to fullfill the brides wishes, she just seems to have to have everything in this wedding so...............go figure......... I just want to make sure i get paid for my work.