I have no idea what is considered reasonable to cover a cake in fondant vs. buttercream. What % upcharge is typical for the market? How do you break it down between simple covered layers and extra embellishments?
We don't charge any more to cover with fondant. The cost for us is about the same since we make our own fondant.
It's not a % more. You have to break down costs and figure out how much more it costs to buy it which should include tax plus shipping. If you are making it then you need to add in the time it takes to make it into that list also.
Break the cost down per pound or 1/2 pound (easier numbers to work with) and figure out your costs from there. That should give you a better idea of how much to add on.
If you want to get really technical you can break it down further and figure out how much storing it is costing you also and add that in.
I charge $1.00 more per serving for fondant. I didn't calculate how more much it cost over buttercream because I make my own fondant so it isn't much more in actual cost and you still have to use buttercream with the fondant. Plus it is just more work!
Local standard for my area is $1.00 more so that is what I charge.