Find another plumber.
He may be right, but I would expect the cartridge in the valve could be replaced for a lot less than that.
Next, if he has to replace the valve, it can usually be done through the tile by removing a minimum of tile. Example: I tried to fix the shower valve in our hall bath and it was so corroded that it snapped off while trying to get it open.
Disaster! Well, not really. I cleaned up the tile and mortar around the valve and called a plumber. He had trouble with it because the master bath sink was right behind. He enlarged the hole in the tile slightly and replaced the valve. The escutcheon for the new valve completely covered the hole in the tile. Total cost was $330 for a Moen valve and labor.
If the standard escutcheon does not cover the hole in the tile, there are oversized escutcheons that will.