Product size: 86*86*25mm, Life time: 2 years (50 calculations per day), Wireless link: BLE4.2, Working humidity: 0%-85%RH, Standby time: three years, Product model: YLKG08YL, Working temperature: -10~+40°C.
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.
Updated:
Banggood
Warehouse in China.
Link: Xiaomi Yeelight YLKG08YL Smart Light Switch
Price: $-3
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Additional Banggood coupons:
1. First page
2. Second page
Country-specific coupon codes:
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – Xiaomi Yeelight YLKG08YL Smart Light Switch [EU] ($-3)
Price history and price alert
Price history (for all/most countries) for Xiaomi Yeelight YLKG08YL Smart Light Switch is here. The lowest price was: $8.58.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
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)).