That $37 water-melon is not for you to take home to the family for a snack. In Japan, expensive, flawless fruit is used as gifts for important formal occasions, such as weddings, or business deals. If you buy it, it will be carefully and beautifully gift-wrapped as well.
Such “gift” fruit is sold among the usual, more reasonably priced fruit in department store supermarkets. If you really just want fruit to eat, there’s plenty available at “normal” prices.
If you want a square/cubic watermelon, that will be about $200.
Japan Tourist Tips 