Latest update shows that SONOFF MINI R4 Wifi Switch Module costs $9.99 (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):
SONOFF MINI R4 Wifi Switch Module Smart Wi-Fi Two Way Switch Smart Home Works R5 S-MATE Wireless Control Alexa Google Home
Banggood ($9.99)
Banggood warehouse is located in China. I’m afraid that there’s no working coupon in Banggood. But wait a sec… Is it a good price? Cool, average is $10.09, so current price is lower!
Link: Banggood product page
Price: $9.99
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
Check out price history chart (for all/most countries) for SONOFF MINI R4 Wifi Switch Module.
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)).