Red caviar is not that expensive, it comes from Salmon.

Black caviar is expensive simply because the fish that it comes from, the best caviar being from Sturgeon, is overfished and underpopulated, and only few places allow the fishing of Sturgeon. Additionally countries like the US outlaw caviar from certain places, also driving up the cost for that specific caviar. There is a hole black market for the sale of black caviar.

Red caviar is more salty and more fishy in taste, it is somewhat slightly reminiscent of lox, just slightly. Lets say if you put lox in a processor , kind of comparable. Red caviar is saltier and not as "fine" tasting, it is not as distinct as black.

Black caviar has a very peculiar flavor, it is usually less salted. The larger the grain of the black caviar the more expensive it is. the smaller the cheaper.