Just check coupons out (with price alert and history). What is best price?
1x Temperature Humidity Sensor, 1x Vibration Sensor, 1x Door & Window Sensor, 1x Water Leak Sensor, 1x Human Body Sensor
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
1. ($59.22): BGSTHO15
2. ($64.67): 7d18f7
you should apply the coupon in your cart after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BGAQFEK8, aff7off, BGaDmITAd, BGBFAFF2019, BGCITYADS2019, Email3, BG9NY, 2020CNY, BGAQ20, BGSAMH, BGAQA15, bGhOnEy10, BGSM15, BGSMOFF, bgshOOp10, BGSAMRTH, BGELIG0920, BgcLuBofErtA10, BG15SH12
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Do you need price history chart (for all/most countries) for Xiaomi Aqara Sensor Kit? There we go.
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)).