For more elaborate gumpaste or fondant pieces (ie: figure modelling, gum paste flowers, custom cake toppers) those should be a cost in addition to price per slice...but figuring out how much to charge for those is entirely up to you. I do believe that I read (and someone correct me if I am wrong) that Lorraine McKay charges a minimum $40.00 for a single fondant/gumpaste custom figure model...so that could be a jumping off point for you.
Hope this helped a little.
Jeanne... great info! I charge by what kind of flower etc...and what kind of detail is in the cake... I don't do buttercream flowers either so not something I can comment on. I am getting more into the gumpaste flowers and have a couple cakes this summer that I am doing the flowers for no extra charge because I want to try them out and told the bride that I won't charge her extra if I can make them for her.. She was excited. I will show the flowers to her ahead of time to be sure it is what her vision is though. The gumpaste flowers are very time consuming if you do them on wires and make sprays so you need to be sure to practice some, figure out your time and charge accordingly.