It is not easy pricing a cake, as you will never receive what you think it is worth. But maybe this link will help. http://www.cakebaker.co.uk/cake-recipe-pricing-calculator.html. It gives you an idea of what you need to incorporate into your pricing and maybe what you have forgotten to include. Give it a try, it may be of some help.
Suggestions for your calculations:
How much were all your ingredients used in this cake (batter, icing, filling, fondant, etc)?
How much of your utilities (e.g. water, gas, electricity) did you use?
How much time did you put in to this cake and how much is your time worth?
Your cake looks good! At the very least, add all the above together. Don't sell yourself short ;)