When I was updating this page the price was $22.99 for Aqara T1 Smart Zigbee 3.0 Light Sensor (in Banggood). Basically copy coupon, view price history or create e-mail price alert. You probably want to know what is the best price?
Latest coupon code price check (UTC time):
Aqara T1 Smart Zigbe 3.0 Light Sensor Auto Smart Home Light Detector Brightness Sensor Magnetic APP Control Work with Alexa Google Assistant Homekit
Banggood ($22.99)
Banggood warehouse is located in China. There are no coupons available at this moment in Banggood. Low/high price? Maybe there’re no more promotions for this item, because current price is higher than average (which is equal $22.05)
Link: Banggood product page
Price: $22.99
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Price history and price alert
Here is price history (for all/most countries) for Aqara T1 Smart Zigbee 3.0 Light Sensor.
Price records source(s): 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)).