I think the Thirsty Gator in Winter Park has the best oysters I have tasted in Orlando. They are constantly shucking so they are fresh and its normally 50 cents an oyster so by far the cheapest I have found. I believe their website is www.thirstygator.com if you need to find them.
Lombardi's Seafood Market has great apalachicola oysters. I know it is not a restaurant, but if you are willing to do the dirty work your self they are great and cheap! 1152 Harmon Ave Winter Park, FL 32789-4994 Phone: (407) 628-3474