Right now the best price with coupon code for HJ813 2.4G is 41.99 USD (in Banggood). Basically copy coupon, view price history or create e-mail price alert. Best price?
Latest coupon and price update (UTC):
Banggood (41.99 USD)
Banggood warehouse is located in China. I couldn’t find any coupon for this product on latest product update. Price? From what I can see this price is higher than average price (41.86 USD).
Link: Banggood product page
Price: 41.99 USD
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BGcd0292, BG5RCCar, BGRCCar5
(there’s a little possibility that old coupon will work)
Aliexpress
Click here… to search for HXJRC HJ813 2.4G 4CH RC Boat on Aliexpress
Here are coupons that you can use.
Price history and price alert
Here is price history (for all/most countries) for HXJRC HJ813 2.4G 4CH RC Boat.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).