Originally Posted by ShandiKakes
Basically the only cost I need to cover is my time.
There's your answer...estimate how many hours it will take you, then multiply that number by your usual hourly wage plus your markup. So if you price your other orders based on a $12/hour wage, you have a 25% markup, and this job will take you 5 hours to complete, you would charge 12 * 5 * 1.25 = $75.
If you don't have an existing business you may want to do some research and see what other cake decorators are making in your area to give you an idea of what a reasonable wage would be.