I’d venture a guess that we are mostly paying for the time and space – for the fish to marinade – labor, and government regulations involved. I’d assume the cost of the fish plays a role as well. There is no fish named herring (or herringus) but thanks for the laugh GH…
Additionally, the companies now produce so many varieties, and creative/artisinal products cost more. If you opt for the plain herring fillets (I forgot the name of the brand, but I saw something similar made by Rockland on Rockland Kosher’s website) you get a lot more fish for your molney. To make it taste good you have to cut it up and finish it off with the flavors/recipe you desire.