Which state are you in? Some states require a licensed and inspected commercial kitchen to legally sell food, while other states have cottage food laws that allow you to sell homemade food.
If you are legal in your area, to come up with a price you first need to determine your cost. What is the ingredient cost for a dozen basic cupcakes, and how long does it take you to make them, including prep, baking, decorating, packaging, and cleanup?
You'll also want to make sure you carefully track your expenses and income throughout the year, otherwise it will be a pain at tax time.