Preheat the oven to 350 degrees F. Line a roasting pan with aluminum foil and coat with nonstick cooking spray.
Place the turkey in the pan and spoon 1 cup grape halves into the neck cavity. Season the turkey all over with the salt and pepper. Pour the champagne into the pan around the turkey.
Roast the turkey for 2-1/4 to 2-1/2 hours, or until no pink remains and its juices run clear, basting every 30 minutes with the pan juices. If the turkey begins to get too browned, cover loosely with aluminum foil.
In a medium saucepan, combine the chicken broth, cornstarch, and the pan drippings with the fat removed; bring to a boil over medium-high heat. stirring constantly until thickened. Stir in the remaining 1/2 cup grape halves and cook for 1 to 2 minutes, or until heated through.
Carve the turkey and serve with the champagne-grape sauce.