You can create price alert here, copy coupons and view price history. Let’s see what best price is ATM.
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest coupon code price check (UTC time):
Humidity measurement range 0.1 (RH), Material: Plastic+Metal Probe, Temperature measurement accuracy 0.1-0.5 (℃), Light test range: 0-2000., Humidity measurement accuracy 0.1 (RH), Size: 26x6cm
Banggood
Banggood warehouse is located in China. Coupons for Soil Humidometer Moisture Meter? Nope, there’re no coupons in Banggood.
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!
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Price history and price alert
Price history (for all/most countries) for Soil Humidometer Moisture Meter is here.
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)).