Aliexpress
Click here… to search for Bardwell F4 AIO RC Flight Controller V2 on Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (34.06 USD): BGAFF10OFF
Coupon (34.06 USD): bGhOnEy10
Coupon (34.06 USD): BGdragonBoat
Coupon (34.06 USD): BgcLuBofErtA10
(CouponsFromChina.com tip: this code has to be applied in your cart after clicking “Proceed to checkout”. Click to copy code and open shop)
Old coupons (from the oldest to the newest): bgssaff9, AFfbG10, BG12MRP, BGVDay10%off, BGSS10%SW, bgshOOp10
(these coupons are old, so probably won’t work)
Price history and price alert
Here is price history (for all/most countries) for Bardwell F4 AIO RC Flight Controller V2.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).