Aliexpress
Click here… to search for Xiaomi Purely Filter Mask Black on Aliexpress
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. You can get coupon down below.
Link: Banggood product page
Coupon (34.10 USD): AFfbG10
(CouponsFromChina.com tip: put item in your cart and apply coupon code there after clicking “Proceed to checkout”. Click to copy code and open shop)
Old coupons (from the oldest to the newest): 935bc1
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Check out price history chart (for all/most countries) for Xiaomi Purely Filter Mask Black.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).