In a large skillet, melt butter over medium-low heat. Add four wontons, and fry, turning often, until they just begin to get crisp, about 3 minutes. Remove wontons from skillet, and transfer to a platter. Repeat with remaining four wontons.
Sprinkle skillet with the sugar, add four of the partially cooked wontons, and cook until sugar begins to caramelize. Add lemon juice, and cook the wontons, turning often, until crisp and generously coated with caramel, 2 to 3 minutes. Remove the wontons to a platter, and repeat process with remaining four wontons. Transfer the wontons to a platter, and let stand to cool.
Place 1 tablespoon of sour cream on top of each wonton, and arrange six to eight grapes on each. Serve immediately.