You can create price alert here, copy coupons and view price history. What is the best price?
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):
Controlled lines: 1, Operating Temperature: -20ºC-70ºC, Wireless Standard: 802.11 b/g/n, Transmission mode: Wi-Fi communication, Voltage Input: DC 5V, Enclosure Material: Fire-retardant ABS, APP Platform: free eWeLink for Android / iOS, Working Modes: APP/Voice/LAN Control, Humidity: 5%-90%RH / Non-condensing
Banggood
Banggood warehouse is located in China. There are no coupons available at this moment 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
Old coupons (from the oldest to the newest): 8Couponsde, 2203ActiesNL, BG00c906, BG589b2e
(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 SONOFF RE5V1C Relay Module 5V WiFi DIY Switch? 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)).