As I point out in my blog article, "It doesn't take a baseball team to decide if you like white or carrot cake." http://cateritsimple.blogspot.com/2009/08/ask-question-how-much-is.html
Plus "decisions by committee" turn into a nightmare.
The Golden Rule: Those with the gold get to make the rules. I offered free samplings which meant I got to pick what was served. I also viewed a sampling as a time when they got to see if they liked my baking .... not as a time when they got to order a sample of everything on my cake/catering menu just so they could decide what they wanted. Not happening.
If it were me ......
"I provide 3 cake flavors of my choosing. Anything over 3 is $25 per flavor/per person."
So 2 add'l flavors with 4 add'l people .... $25 x 2 flavors = $50 x 4 people = $200. They are not paying for THE CAKE. They are paying for the xtra time you have to take out of your VERY BUSY DAY to bake this extra cake for them. Time is money .... if they want your time, they have to fork over the money.