I think it's 10 or 15$ for each one. And I'd assume there's the usual 10% AP discount. Never done it myself.
Don't forget, they also have the oyster tanks in Epcot at the Japan Pavillion. Of the three places, I think getting one at Sea World would be the coolest. As there are actual divers in a huge tank. You point to the location, and then they'll pick up a handfull to bring to the surface, and you choose one. There's nothing special about DTD, and Japan has a small tank going around the counter with oysters to choose from.