I am just starting my little at home business and when people call or ask me what do I charge I really dont know how to answer that! I then end up undercharging for something that took some time to do!
Go to www.earlenescakes.com and click on the Cake Serving Chart button to seee how many servings per specific cake size.
As for prices, it depends on your area, but most of the people on this site think you're nuts if you charge anything less than $2 per serving (buttercream), and even that may be a little low. Fondant seems to start at $3 per serving. Items such as time consuming flowers and fondant figures are extra as well. Click the search tab and enter price - you will get quite a few previous discussion threads on the topic.
Also, call some local bakeries and ask what they charge to see what your towns going rates are.