@Titen
just googled it and apparently, watermelon has always been on the pricier side in Korea. Currently, the average retail price is $19 (USD) for a watermelon.
China's watermelons are probably very cheap, because they are the leading producer of watermelon in the world. Last year, they produced 63 million tonnes of watermelon, which apparently was 60% ofthe world's production. The next leading producer is India with 3.6M tonnes.