Nintendo is facing Ring Fit Adventure shortages around the world, but things are especially bad in Japan. The package is quite limited due to production issues, which stem from the coronavirus outbreak in China. While Nintendo waits for more stock, people have been scrambling to get their hands on whatever stock is left.
It shouldn't be much of a surprise, but scalpers and resellers are milking the shortage for everything it's worth. The package can be found on reseller sites right now at over 3 times the regular retail price, and those auctions are moving. As the situation only gets worse, expect the price to continue to climb.