How to Sync GroupBox and ListBox?

Hello, I'm very new at C # and have to come up with a pizza shop application. The pizza pies have different prices based on the size you want. The best I've come up with for a layout was to have the pizza types in a listbox and then have a groupbox with radio buttons for the size. They can only choose one pizza and radio button. I am not sure how to compute the costs. For example a Plain pie costs 1.50 a slice, 8.00 medium, 10.00 large. There are 7 types of pizza and the 3 size types. I'm thinking of using if..else statements but not sure how to do it and it's driving me crazy. I don't have any hard code just the windows form layout done. If there are any suggestions I'd really appreciate it. Thanks a Lot.