I haven't charged anyone for a round cake yet, but when I do, I will charge $X.XX per serving. That way it won't matter if they want an 8" on top of a 12" or (2) 10" etc. Just figure up the servings the cake will provide and multiply times the cost. I will have a 'basic' rate for cakes with minimal or some decoration, and a 'premium' rate for cakes with lots of detail work involved. (Now I just need to figure out my costs so I can hammer out the details! LOL)
Hope this helps a little. Maybe someone with more experience will post and help us both!