They broke the oil cap it should be their expense to fix it. It's not uncommon for the oil cap to break. Even so you do not have to buy a new cover. A good tech can get the old oil cap off.
Now if the dealer drained the oil without removing the cap first and they are using this as an excuse that they can not put oil back in it. They can remove the cap without buying a new cover. I'm sure if they can not remove the cap then tell them to get a used cover. I think somebody is trying to generate sales at the dealership.
If they do not see it your way call the 800 number corporate and complain till you get the answer that is acceptable to you. Good luck