IIRC, a damaged stud is NOT covered under an extended warranty. Studs do not fail by themselves at that mileage. They fail from the repeated removal and installation of a lug nut during a tire rotation which is part of routine maintenance. Sometimes they are torqued too tight, sometimes they aren't, sometimes they are threaded wrong, who knows. Welcome to owning a vehicle and the costs involved of owning one. I would never even attempt to try and get that covered. I have had to replace a few studs on my vehicles and it is very easy to do especially with a c clamp(screw the washer and nut method which damages threads on both the new lug nut and stud anyways)
Now with that said, I think that is a little high for a replacement. $50-60 is more reasonable. My philosophy is this. If my technician breaks a bolt/lugnut/etc when we are taking it off and we have not touched that before the visit, the customer is on the hook for the repairs. If we break it putting it back on or we have removed that in the last visit, then we are on the hook for the repairs.
Is this the same dealership you have repeated problems with in the past?