Whenever I go for quest fish I catch a whole bunch of them, often one right after another. They're not super rare. You just need decent equipment, a sufficiently large pond/ocean/honey pool, and a little patience.
With a good rod and average bait I have no trouble at all bringing in five quest fish (one to give to the obnoxious kid and four to store in a chest for later) plus heaps of seafood, potion ingredients, tool-fishes, crates, and the occasional Frog Leg, Zephyr Fish, or other special item. It's not unusual for my characters to fill their inventories up and have excess Reaver Sharks, Sawtooth Sharks, and Swordfish literally covering the floor around them.
After selling what I can't use my average fishing trip generates between one-half and one platinum, plus the crates which I'm keeping to be opened in hardmode where they'll be worth more.
[edit] Ack, thread necromancy. The OP's probably no longer around to read this. Maybe it will help someone else.