My experience is the higher level worlds generally have rarer and somewhat more expensive fish. My balance wizard made over 50k in his first week from fishing every day.
Overall a good sum of that was from boxes, but he also made a good bit on fish of the day catches which improve 100 gold per day (meaning if he catches fish of the day 5 days in a row they are worth 500 more each), but if memory serves I have sold some single fish for over 1000 gold.
It really depends on the type and what is the Catch of the Day and a numerous of other items. Remember no two fish even of the same type are worth the same value.
