Aliexpress
Click here… to search for Mini Digital LCD Indoor Thermometer Hygrometer on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. There is one great coupon!
Link: Banggood product page
Coupon (8.82 USD): BGherra
(CouponsFromChina.com tip: just apply this code in your 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): 15ANENG, TTxBG10, aff7off, bgssaff9, AFfbG10, BG3338, BGBFAFF2019, BGCITYADS2019, BGVDay10%off, BGAFF10OFF, bGhOnEy10
(these coupons are old, so probably won’t work)
Price history and price alert
Check out price history chart (for all/most countries) for Mini Digital LCD Indoor Thermometer Hygrometer.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).