Model DEM-F420, Material ABS, Color White, Frequency 50Hz, Decibel ≤36 db, Voltage 220V, Timing 12 Hours, Application 31-40 ㎡, Power 20W, Capacity 4L, Humidifying Capacity 280 ml/h, Weight 1.1kg.
Basically copy coupon, view price history or create e-mail price alert. Let’s see what best price is ATM.
Aliexpress
There’re some similar (or not) items on Aliexpress
You can use link bot to get even lower price:
Click here to search by yourself. Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
1. ($29.18): BGDEER05
2. ($31.19): PICODICOM
you should apply the coupon in your cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/i1u
Old coupons (from the oldest to the newest): 502C6B, 919c60
(there’s a little possibility that old coupon will work)
Price history and price alert
Here is price history (for all/most countries) for Xiaomi Deerma DEM-F420 4L Air Humidifier. The lowest price was: $28.51.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).