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 (286.99 USD): BGbe3547
Coupon (373.09 USD): Cuponation9
Coupon (373.09 USD): BgGoodie9
Coupon (377.19 USD): BGJON8
Coupon (377.19 USD): BG8ReviTrage
(CouponsFromChina.com tip: coupon code has to be applied in shop’s 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): BG62eb59, Drones6, BANGGOODSD, CouponsDE, 00c906, Tabby20, CouponsdeFPV, 8Couponsde, MVC8SPRING, 8BGwbg2204, tRaDetRaCKeR, 8wEbgaInS5, BG5b2913, pROMo8cuPoN, BG8RCToy, hoNEYbGdeAlS, BGJUNEGSG9, BG874251, COUPONSDE1wk
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Price history (for all/most countries) for BetaFPV X-Knight 35 4S Polar 155mm Drone is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).