The most common extra that is asked about is rolled fondant. A fondant covered cake will add $.75 per slice.
A close second is specialty icings and fillings. This includes fresh fruit fillings which are $.75 per slice, and whipped or regular ganache that run $.50 per slice as a filling and $1.oo per slice as an icing.
Also very common, are elaborate piping designs on a butter cream frosted cake. This cost will vary depending on how elaborate your design is. All cakes without fondant come with a top and bottom piped border on each tier free of charge.
Many people like to also have flowers or decorations sculpted out of Gum Paste. These are all priced per request due to the fact that there is such a wide variety that can be done, all varying in difficulty.
There is also the option of doing sculpting withmolding chocolate, which also varies in price due to the same reasons as the gum paste.
Fresh flowers, ribbons and other items that need to be bought from store are the cost of the item plus an added arrangement fee.