Over looking the copyright issues...
An 8in square cake would serve 32 and an 10in square cake would serve 50. So I would price it based off of those two sizes, not just 25 and 50.
Your base cake price should be off how many it serves, not how many they want it to serve.
I'm not a big up-charge for a design person, but there are times when it is warranted. This would be one of those times. I would do a basic fondant cake price (use the link above to get those numbers) and then I would tack on time and supplies for all the extras.
No one can really tell you how much to charge. You have to make sure it covers YOUR costs, time, not to mention profit. There are a lot of things that fact in to costs. It may cost one person $30 to make a cake and cost the next person $50.