I was advised by many experienced cake makers that prices should be based on basic costs plus time and additional materials used. I worked out how much a gram of sugar, flour etc costs. How much one egg costs and so on. It takes a while to do this but it means that every time you do a cake all of your basic costs are then covered. Obviously include cost of icing per gram, basic cost of cakeboard, and what you are going to pay yourself for your time.
That should give you basic costs for each cake size and shape. Then you decide what your profit margin is to be. I know this sounds like a long process, but it will only take a couple of hours, then you have a full price list for any cake you do, unless its out of the ordinary :-)
