"How much should I expect to pay for a cake?"
Cake prices will vary according to demand, geographical location, expertise of the decorator, by how complicated the cake will be to assemble and decorate, and quality of ingredients used to create your cake.
Demand will often drive the cost of a cake up or down. For example, if a new bakery is trying to establish a name for themselves they will keep their prices low to entice buyers to try their product. A company whose business is exceeding their competitors by a wide margin will increase their price simply because they can without their business suffering a loss. Look at the prices of cakes at Charm City Cakes or CakeLove before and after their owners had popular shows on the Food Network and their prices have likely increased considerably.
Location will affect the price you pay. Prices are higher in New York City vs lower in Bismark, ND not because the cakes are necessarily better in NY but simply because the overall cost of owning and running a business is higher there than in the Dakotas.
Expertise. The more knowledge your cake decorator has about cake decorating, the more you're going to be satisfied with the quality of work they have performed on your cake. Lines will be more precise, buttercream will be as smooth as glass, fondant will not have seams, cracks, or wrinkles, and you'll never worry about whether or not your wedding cake is going to cave in on itself.
Complicated assembly costs more because it takes more expertise to create. A single-layer sheetcake that your auntie can make in her kitchen can be made by... well, your auntie. If you want a topsy-turvy cake or one that's built to look like the Guggenheim Museum, that's going to require quite a bit more planning than your auntie's cake that is served out of the 9x13 pan.
Quality of Ingredients is, in my mind one of the most important aspects of cake design. Sure, people will ooh and ahh and ask, "How could I ever eat such a beautiful thing?" but when it comes right down to it, people buy a cake to eat it. True, some people will swear that 'box' cake is their favorite and many wonderful cakes have come from box mixes but I believe that a truly ethereal cake must come from the highest quality ingredients that have been lovingly assembled into something amazing.
All of these aspects can come together for just a couple of dollars per serving for a beautifully (albeit simply) decorated cake for a child's third birthday or they can jump beyond $10 per serving for an elaborately decorated, multi-tiered wedding cake.
If you talk to a bakery about your cake idea and they quote a price that you feel is too high, it probably is. Look around at other bakeries in your area and don't forget to check out the grocery store bakery aisle -- truly, this is a way of getting your berings if you become lost in a sea of ideas and prices. Grocery stores will make a cake that is just a step (maybe two) above your auntie's 9x13 cake mix cake with very little lead time (grocery store bakeries only require an hour or two to complete a decorated cake because their cakes are made ahead and frozen until you order), this way they can keep costs low.
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