Offline
I have a basic example.
I bought 5 F baits. They're cheap; The total price is 125g.
I went fishing in Durem, and did so one after another. Now, by the last one, my luck wasn't very good. However.
I got 930 by selling my fish back (including some from moving from Durem to the fishing shop).
Even if you strike the original 125 off, that's 805 gold in profit.
The Basic rod will be payed off in one bucket of fish.
The Distance rod will be payed off in about twenty buckets of fish.
The Strength Rod will be payed off in about eight buckets of fish.
The Performance rod will be payed off in about fifteen buckets of fish.

That means that if you fish for about an hour or so per day, you can get the gold you used to get the rod in less than a week.
I bought a Strength Rod + from a guy for 10k. Right now, I have just under 7k, and I've been donating 200-500 gold in chunks to people every now and again.