The Catholic church has a rule not to eat meat on Friday and fish was popular instead. Not sure when that started but maybe forcing a bit of more expensive fish into the diet was quite useful for dietary health.
As I understand it, meat (non-fish meat) was considered a luxury, not the other way around. But I'm not 100% sure of the context.
Before refrigeration and the steam engine fish prices would rise dramatically and quality decrease quickly as you move away from the ocean. Salting was done to preserve some types of fish, but this processing would add cost. As discussed elsewhere, people inland are the ones who would lack iodine in their diets and in those locations and I would guess fish would be the more expensive than meat in those locations. It would be interesting though to see a list of market prices for meat and fish from some inland city back then.