Aliexpress
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (61.61 USD): BGAFF10OFF
Coupon (61.61 USD): bGhOnEy10
Coupon (62.98 USD): Parts8
(CouponsFromChina.com tip: this code has to be applied in your cart after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): F438, 502C6B, AFfbG10, BGVDay10%off, 2880c7
(there’s a little possibility that old coupon will work)
Price history and price alert
Here is price history (for all/most countries) for 20x20mm HGLRC XJB F438 RC Flight Controller.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).