Your expectations seem low when you ask about the price of burgers, fries, and pizza. Seaworld and Busch Gardens have a much greater variety of food than that. You will find many types of sandwiches, interesting sides, and various entrees that don't fit the sandwich category such as bbq chicken, ribs, pasta dishes, Asian food wraps, salads, etc. The park's websites have menus and I suggest checking them out under the dining category on each park's site.
No matter what you choose for food, the prices tend to be around $10 for a sandwich and side, $10-15 for other meals, and around $3 for a drink.
To give you an idea of the menus, here are many photos of the menu boards at Busch Gardens Williamsburg. You will find the food styles and prices very similar across all of the Seaworld owned parks.
http://forum.parkfans.net/thread-1847.html