Why Uni is so expense? Well, there are a lot of reasons.
First of all, there are no viable Uni farms, so they all wild. Uni is very fragile and very influenced by the water and climate so the best tasting Uni is specific to a area like Hokkaido.
in Hokkaido, all Uni is harvested by hand from the wild. Uni also must be prepared by hand, there is no mechanical method of cleaning and preparing the gonads for consumption - Uni has only 5 small edible parts.
In the meantime, Uni begins to deteriorate immediately when it's exposed to air. Uni even an hour or so old has a distinctly different flavor than freshly harvested. In another word, it must be processed, packed and shipped immediately by the fastest methods available.
You have to eat it very fresh which means it is flown overnight to market and then distributed very quickly. Freezing Uni is not impossibly but not so good idea when you want to serve it to your guest because it won't preserve the flavor well and radically alters the texture.
On top of that, there is fierce competition for truly outstanding Uni. Many chefs who know good Uni but are not always at the top of the list won't serve Uni of lesser quality.
All those thing attribute to make Uni price so high, contrary to expectations.