. . . cod is a luxury good?!
Quite the f--king opposite, mate.
Actually, cod was a very valuable commodity in those days. While not in the same league as spices or silk, fish were good income generators. Remember, Europe is Christian and that means the only meat you can eat on Lent and, under most traditions at the time, Fridays and Holy Days are fish. Cod salted well which means that you could keep it around for months and thus a good commercial fish in the days before refrigerators or ice makers.
One thing we don't appreciate in these days of global supply lines and industrial farming is how valuable 'basic' food items like beef, fresh fruits, even basic grains that aren't locally grown were back in the past.