Aliexpress
Click here… to search for Digoo DG-C5 Hygrometer Thermometer on Aliexpress
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. Well, there is one coupon available right now.
Link: Banggood product page
Coupon (9.34 USD): BGAFF10OFF
(CouponsFromChina.com tip: apply code in your basket (cart) after clicking “Proceed to checkout”. Click to copy code and open shop)
Old coupons (from the oldest to the newest): TTxBG10, BGss8%hl, bgssaff9, AFfbG10, BGC500, BGDigoo15%, BGMC547, BG7C5T
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Price history (for all/most countries) for Digoo DG-C5 Hygrometer Thermometer is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).