Just check coupons out (with price alert and history). Let’s see what best price is ATM.
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest price update (UTC time):
Humidity range 0%一99%RH, Temperature display resolution 0.1°C, Measure range 0°C-60°C, Executive standard QB/WSDJ 2401-2019, Battery CR2032, Wireless Connection Bluetooth 4.2 BLE, Brand XIAOMI Mijia, Humidity display resolution 1 %RH, Material ABS+PMMA, Quantity 1Pcs, Model LYWSD03MMC, Size 43x43x 12.5mm, Voltage DC2.5V- -3V, CMIT ID 2019DP8115
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
1. ($18.99): BGTYOI324
2. ($18.99): BGPS324
3. ($25.73): 00c906
this code has to be applied 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/5cd
Old coupons (from the oldest to the newest): BGMAYTD324, BGwhielabel, BGYFO24, BGISXIAOMI1, BGSPXIAOMI, BGPT324, BGJUTD324, BGdc324, BGXMFO324, BGPDhome53, BG10OFFAFF1, BGTYOC324, BGTYC324, BGISOC324, BGAUC324, BGAUOC324, BGTYOG324, BGSPHOME41, BANGGOODSD, BGUdb55, BGUbf50
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Do you need price history chart (for all/most countries) for 4x Xiaomi Smart Bluetooth Thermometer Hygrometer LYWSD03MMC? There we go.
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)).