Depending on the extent of the overheating that would have caused the gasket breach in the first place, milling isn't always needed.
But as said, independent shops can do this for far less than the dealership.
I was quoted $100 additional for the service (320, not 420 like yours). It turned out that it didn't need milling, but got the head components replaced or reconditioned. If the dealership opts to double repair costs, they'd should at the very least completely recondition the heads!!
This was mine after it came back from the machine shop:
