What are you charging for a basic sheet cake per serving.
I charge $1.75 per serving where I am ($2.08 us dollars)...that's for basic buttercream icing and decor. I go up from theta. Hope that helps!
Information on how to price a cake: http://cakecentral.com/cake-decorating-ftopict-694973-pricing.html
Serving amounts: http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm
A single layer 13x9 serves around 25 1x2x4 in pieces. Double layer serves 50.
For me buttercream cakes are $3 a serving, so $75 for the single layer.
Ditto what TexasSugar quotes....
I charge the same for sheet cakes as I do for wedding cakes - per serving. If it's a single layer or double layer, it's all per serving.