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To price the cake, again you would need to know your ingredient costs, labor costs, and overhead. Let's say the ingredients for a half-sheet cake (12"x18") cost $30, your "wage" is $15/hour, and it takes you 3 hours to make and decorate the cake: this adds up to an ingredients & labor cost of $75. If your annual overhead costs (including utilities, liability insurance, licensing fees, and commercial kitchen rent) come out to $8,000, and you make 100 cakes per year, that's $80 in overhead per cake. Add these up, and your cost (not the price) is $155. Add a 20% profit margin to get the price and you're at $186. If a half-sheet double layer cake serves about 50 people, that's ~$3.75/serving.
It's extremely important to have your costs down before you start selling, otherwise you could end up losing a lot of money.