I went to a number of local places and they were all about the same as this. I even went to a fish and chips place in Reykjavik expecting it to be cheaper since they certainly don't need to import fish, but it wasn't. Another tiny family-owned restaurant in Vik was the same as well. I honestly couldn't find cheaper food.
I do understand the difficulty in importing food to an isolated island with only 100k people. I'm just not sure that fully accounts for the increasingly high food prices across the board.