Prices vary depending on which fender you get, from whom, and whether or not you need to paint/repair it. Many times depending on the damage they can be repaired.
For the dealer, if you have before and after pictures hard for them to deny it. If they have a walk around and indicate no damage, I'd go after them. "That would have been easy to miss in our initial walk around" doesn't put the burden on you. They're the dealer, they should know this and they should inspect that area before hand.