I have my pricing down, so I'm not asking how to price my cakes. I do however have my "specialty" cakes priced per serving. I've been doing some browsing through old threads and it seems some price theirs by the work/hours it takes them.
My only concern with pricing per slice is really not knowing how many slices someone would get out of the cake. For example, I did a hula girl cake (not for selling), so I used a 11x15x2 but hacked off sides of the cake to give it an "hour glass" shape, and then added "bobbies" to the top of it. I know how much the 11x15x2 serves full, but not hacked away at. So it got me thinking I might want to change my way of pricing those types of cakes.
So the survey is...do you price your specialty/3D cakes by the piece or the work/hours it takes?
AFor any cake that is sculpted or 3D the number of servings has far less of an impact on price than the labor required. In some cases a smaller cake with a specific design will be the same price or more expensive than a larger cake with the same design if there are fine details involved.
Of course you still need to know how many people the cake will serve, a good way to do this is to form the remnants into a "cake" and subtract the number of servings in the remnants from the number of people the pre-carved cake would serve.