Aliexpress
Click here… to search for Xiaomi Deerma DEM-F450 4L Air Humidifier Black on Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. Only one coupon available!
Link: Banggood product page
Coupon (34.02 USD): bGhOnEy10
(CouponsFromChina.com tip: apply code in your basket (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): 502C6B, bf18a5, cf38bb, 511a5d, PICODICOM, 89de58, 919c60, BG209FC, BGDEER05, BG2099F450, bgssaff9, BG19demf450, BGHU283, aff7off, BG20F450, BGaDmITAd, BG9F450, AFfbG10, BGBinkupon501, BGDEMF450, BGCITYADS2019, BGDER450, BGDEM450, BG23DRA, BGDF450, 2020CNY, BGVDay10%off, BGDEM45, BGHOMEAPP
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Here is price history (for all/most countries) for Xiaomi Deerma DEM-F450 4L Air Humidifier Black.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).