I have a pricing grid/matrix - I have 4 levels: Basic, Level 1, Level 2, Level 3. 3 icing choices: Buttercream/Poured fondant/Rolled fondant. And finally amount of servings: up to 100, 101-150, 151-200, Over 200.
The most expensive per slice cake is a Level 3 cake with rolled fondant and serves under 100 - $12 per slice.
The least expensive per slice cake is a Basic cake with buttercream and serves over 200 - $2 per slice
My definitions of levels:
Basic - BC: Top border, bottom border, party confetti/small floral spray/fleur de lis and/or a message written on top
Poured Fondant: Ganache over BC with bottom border and a few BC accents or premade gumpaste accents
Rolled Fondant: RF over BC with bottom border and a few premade gumpaste accents or a BC cake with a few Fondant accents
Level 1 - BC: any basic side piping (swags, swiss dots, corelli lace, sotas); PF: Fresh fruit garnish, any of the BC elements that make it a Level 1. RF: a few (no more than 3) handmade gumpaste flowers or a basic fondant swag.
Level 2 - BC: slightly more intricate piping or architecture, PF: same, RF: same and more gumpaste flowers or more intricate gumpaste work
Level 3 - BC: includes Lambeth method, string work, and all topsy turvey designs and 3d carved cakes, PF: same, RF: same
You will have to decide for yourself what constitutes the level breaks for your designs. I do give those who have bigger cakes a bit of a price break due to the volume, plus I rent a comm kitchen so the larger the cake the better value I get for my rental rate than if I have a smaller cake. I pay a flat daily fee of $100 and I get the kitchen for 24 hours, so a smaller cake will cost me more to produce than a larger cake. I pass the savings on to my customers.
This way you aren't starting with a base per slice rate then adding on embellishment charges which will arrive at the same per slice rate anyway, but to the customer, it looks like they are being nickled & dimed with each additional charge. If you quote them the per slice rate for their design choice and serving amount off the bat they think they are getting a deal. When you put that low ball per serving rate in their head it is hard for them to go beyond that. It is all about perspective.
If you like I can PM you my grid, but you will have to decide what works for you price wise for your area. You may be in a very high dollar area or just the opposite.