Hi there :)
I use to try and price my cakes upon a 'per slice' basis.. but it doesn't make sense really, as you most likely know already that every cake you design and decorate is so uniquely different and there are so many variables.
I feel personally that the best way is to cost out your ingredients, materials, gas & electricity, water (to the very last.. including the ribbon around the base board!), and then sit down and really try your hardest to guesstimate how long this cake is going to take you to complete.. from getting your ingredients out of the cupboard, preping, baking, ganaching, covering, constructing together the tiers to finally decorating! Once you have worked out your hours of labour, you need to ask yourself what you feel your time is worth per hour. I for example try and work for $25 p/h.. it doesn't always work that way.. and I know as an artist I get carried away and feel the desire to go 'above and beyond!' lol.. and if I do, well hey that is my choice if I don't charge for it.
My calculations would be for example a cake that was going to take 15 hours to put together and cost $125 in materials and ingredients etc -
15hrs x $25 = $375 + costs $125 = $500
And I am a BIG endorser of the pricing software CakeBoss.. omg it is invaluable when you are serious about running a professional business and ensuring you are not ripping of either yourself or your clients ;)
Oh and I definitely torte my cakes.. twice actually.. I actually thought most Aussie decorators do?!