Basically copy coupon, view price history or create e-mail price alert. Let’s see what best price is ATM.
Max. power: 2200W / gang, 3500W / total, Max. current: 10A / gang, Power supply: 90 – 264V AC (50 / 60Hz)
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 and price update (UTC):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
1. ($17.49): BgcLuBofErtA10
2. ($18.43): 13f142
put item in your cart and apply coupon code there after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): 502C6B, elec, BG4CHR2, BGCITYADS2019, BGE13, bGpAnDacHeCk, BGVDay10%off, 0e7f33, FBCOUPON1, bGhOnEy10, BGTT10, BGSMOFF, BGSONOFF15, BGELIG0920, BGSONOFF16, BGSHOM15, BG15Sonoff
(there’s a little possibility that old coupon will work)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – SONOFF 4CH Rev2 Smart Switch [EU/UK] ($-3)
Price history and price alert
Price history (for all/most countries) for SONOFF 4CH Rev2 Smart Switch is here.
Price records source(s): Gearbest, 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)).