Is there any of the stud still sticking out from the head, or is it flush or inside? If there's none sticking out then you'll need to drill it & use an "easy out" Be sure & spray the heck out of it with WD-40 or some other penetrating lube a few times before trying to remove it. The reason these snap off is because of the length of the exh manifold allows it to warp outward with all the heating & cooling cycles & pop the stud(s). You will need to have the manifold machined back to true. I have no idea how much this would cost at a shop because labor rates vary so much across the country.