Material: PC V0;, Wi-Fi: IEEE 802.11 b 2.4GHZ;, Quiescent current: ≤ 40 μA;, Not included);, Working Voltage: DC3V (2x 1.5V battery, Battery Model: AAA 1.5V;.
Basically copy coupon, view price history or create e-mail price alert. Best price?
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 coupon and price update (UTC):
Banggood
Banggood warehouse is located in Europe 
. Hmmm… There is no active coupon in Banggood at this moment. I’m not sure about the price, coz I have only 1 record(s) in my database.
Link: Banggood product page
Price: $-3
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Additional Banggood coupons: first page and second page
Old coupons (from the oldest to the newest): BGSONOFFDW2, BGSONOFF15, BGELIG0920, BgcLuBofErtA10, BGDW2, BGSONOFF16, BG1674, BGSTHO15, BGHONEY2110, BGSHOM15, BGSHE15, 13f142, 6GSGIT, ADMITAD2212, BGSmart6, MVC8SPRING, pROMo8cuPoN, BGJUNEGSG9, BG15Sonoff, BG8b589f, Cuponation9
(there’s a little possibility that old coupon will work)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. China – SONOFF DW2 Door/Window Sensor ($9.59)
Price history and price alert
It’s a new item in our database, so I can’t create price history chart. We have saved this price in database: $9.99.
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)).