Aliexpress
Click here… to search for HGLRC XJB F428-TX20-ELF F4 RC Flight Control System 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 (96.06 USD): 7bcdf9
Coupon (96.66 USD): Parts8
(CouponsFromChina.com tip: apply it in your cart after clicking “Proceed to checkout”. Click to copy code and open shop)
Old coupons (from the oldest to the newest): 502C6B, f62884, BGss11, BGPAYPALSS, BGBCFPV, BGBCBG10
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Here is price history (for all/most countries) for HGLRC XJB F428-TX20-ELF F4 RC Flight Control System.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).