Just check coupons out (with price alert and history). What is best price?
Working voltage 3V, Green light Suitable for moisturizing, Red light Water shortage, Weight 30g, Working current 10uA, Size 131x 25.1x 10.9mm / 5.16x 0.99x 0.43inch, Rated power 0.03W, Battery 1x 3V CR2032 button battery (not included), Color White, Product name Flowers and plants detector, Blue light Too much water, Material Shell ABS plastic + detection sheet material-glass fiber epoxy copper clad board, 05, Measurement accuracy -0
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. ($9.99): BGcaa3e6
2. ($13.80): BG1efeb0
3. ($13.95): Terence
4. ($13.95): 2203ActiesNL
5. ($14.10): BG00c906
apply code in your basket (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): BGOOR3WW, BgcLuBofErtA10, BGZKHDW1, BGTEST37, BGAFF10OFF, BGGDJUT7, BGTESCN02, BGHONEY2110, BGAP2803, BGwhielabel, 13f142, BGAU0407, BG10OFFAFF1, BANGGOODSD, 00c906, ADMITAD2212, BGAlerabat1
(these coupons are old, so probably won’t work)
Price history and price alert
Check out price history chart (for all/most countries) for Smart Sensor Plant Flower Monitor.
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)).