Size matters, right? Is it to be 10 feet tall? Six inches? It sounds to me like you may possibly be getting ahead of yourself in selling cakes if it didn't occur to you to include size in this question. The specific design, level of detail both come into play. And perhaps WHICH large figure.
Check out the Pricing Formula link in my signature below for help on calculating a price based on your cost structure and market value.
Note that the Despicable Me characters are copyrighted so you would need permission from Universal before you can recreate the characters in fondant. If you are not able to get permission an alternative would be making a generic cake with the same color scheme and purchasing licensed figurines for the top.