The way I price my cakes (3D or otherwise) has nothing to do with price per serving. I add up all my overheads (ingredients, power used, boards, dowels etc etc), then I estimate how long it will take me to mix ingredients, ganache the cake, cover in fondant and decorate. I give myself an hourly rate and multiply that by the time it will take me to do (Eg $25 x 8hours = 200 ). Then add my overhead costs on top of that ($100). I now have a price ($300). Those prices were just examples, but I always tell my customers that the size of the cake isn't the determining factor of cost. Some small cakes can take much longer to decorate than large ones if the design on them is intricate... or you need to hand carve it, etc. These styles of cakes are art work... if they wanted the type of cake where they paid by the serving they'd go to a bakery and buy a sponge cake..
I used to undercharge myself drastically in the beginning, but after you've been doing it for a while and you realise how much work goes into every cake, you'll quickly stop doing that, haha.