assuming you have art 2 trade, a sailboat and max inventory AND you want to upgrade to a Frigate for the sole purpose of hotspot fishing.
filling an inventory costs 3 hours and 3 triple float rods (9mil x 3 = 27mil) yes you might be able get triple floats for free from quests but they are still worth something.
trade in = approx 130mil - 27mil of rods, lets just call it 100mil in 3 hours
lets say the frigate will save you 30 minutes over a sailboat bringing fishing time down to 2hr 30min to make 100mil.
how long before you see a ROI? (Return on investment) for hotspot fishing only?
Sailboat = 33mil an hour
Frigate = 40mil an hour
difference = 7 mil an hour
7mil x how many hours? = 2bilion
285 thousand hours or about 12,00 days