Aliexpress
Click here… to search for Digoo DG-HOSA Smart 433MHz Smoke Detector on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. I found coupon for you. Check this out.
Link: Banggood product page
Coupon (13.02 USD): 6GSGIT
(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): aff7off, BGaDmITAd, bgchinafest, BGBFAFF2019, BGCITYADS2019, BG9NY, bGpAnDacHeCk, 2020CNY, BGVDay10%off, BGDG15, BGSAMH, 20%DG422, BGDigoo15%, BGSMOFF, BGDGASG, BGSAMRTH, BgcLuBofErtA10, BG15SH12, BGSTHO15, BGHONEY2110, BGSHOM15, BGSHE15, 13f142, BG0a05ae
(there’s a little possibility that old coupon will work)
Price history and price alert
Here is price history (for all/most countries) for Digoo DG-HOSA Smart 433MHz Smoke Detector.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).