According to Official Guinness Records,
The most expensive watermelon sold at auction is CNY 51,000 ($7,489;£5,827;€6,442) and was achieved by Inner Mongolia Green State Fertilizer Co., Ltd. (China) in Kerchin, Inner Mongolia, China, on 26 August 2018. The watermelon weighed 81.75 kg (180.22 lb).
For a complete list of 2018 records, please visit 2018 Guinness Records in China.