I’m guessing you’re talking about Tobiko, which are fish eggs widely used in sushi:
Ikura, or salmon roe:
Beluga caviar, on the other hand, comes from a highly rare and critically endangered fish.
Salmon can be found in plenty. There are a lot of flying fish. Sturgeon, particularly the enormous ones that produce beluga caviar, are quickly going extinct. (Other, less costly sturgeon caviars from less-endangered types are available.)
As a result, beluga caviar is exorbitantly priced (and bad for the species). The rest may be had for a low price (and as guilt free as anything can be that requires killing animals).
Demand and supply are the two factors at play.
When there is a scarcity of anything, it might lead to a surge in demand, which raises the price.
Beluga caviar is produced by a single species of slow-growing, slow-maturing fish that can only be found in a tiny part of the planet. The majority of fish are slaughtered to collect the caviar, thus diminishing the nearly extinct population and increasing availability and price.
Sushi is the Japanese word for fish. Any variety, with the more sustainable and abundant types being the most affordable. Some of the more uncommon varieties may be rather costly… The word “blufin” conjures up images of bluefin tuna.