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;.
Go down and check coupons out. Remember to check price history if it is available. You probably want to know what is the 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 code price check (UTC time):
Banggood
Banggood warehouse is located in Europe 
. Too bad there’s no coupon right now in Banggood. I don’t know if this is a good price – number of records in my database is too low!
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
(these coupons are old, so probably won’t 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
Not enough data to show price history chart… $9.99 is the only recorded price. It was not cheaper than that since we have saved it in DB.
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)).