Your cake per person cost is based on the difficulty of the cake design. A simply decorated cake with fondant or butter cream icing will be in the $8.50 range. More decorations, colors, etc. will up the cost. All fillings and sponge cake flavors (except fruitcake) are figured into the price. Disney does a per person price to insure there will be enough cake for all of your guests. If cost is a concern, ask about having your cake design done in the minimum guest count (50), and have an undecorated sheet cake retained in the kitchen that will be cut up and served with the displayed cake.
After checking with some area bakeries (Orlando), and some more famous cake decorators around the U.S., I see that Disney prices are at, or below the average. Keep in mind that the wedding cake department does ONLY wedding cakes,and have been for almost 10 years.Your end result will be excellent.