It does pay big time but they can be very tedious to get enough jellies for, especially if you’re in advanced game and reached level 10 the jellies are super rare then. And you need one of each per machine. But once you have about 10 or so they really help make fishing profitable and double the price of any fish. They do also require 1 coal per fish so keep that in mind if you buy coal. They process fairly quickly, though I always process expensive fish first and then work my way down. Often considering they use 1 coal I still process less than 75 cent fish as sashimi to eat or put into shipping bin for a touch of profit without using precious coal.
WARNING! Do NOT process a Legendary fish. Save those for a fish pond and get tons more of profit that way forever. Plus if you are in advanced game and unlocked mastery cave farming one you can get occasional golden animal crackers and they can be placed into fish pond with legendary fish for double the profits. Way profitable!