$16.99 – Model: D1, RF band: 433.92Mhz, Enclosure material: PC V0, Output: 100-240V AC 50/60Hz, Input: 100-240V AC 50/60Hz, Dimension: 62*47*32.5mm, Wi-Fi: IEEE 802.11 b/g/n 2.4GHz, Dimmable LED: 150W, Incandescent bulb: 200W, Max. current: 1A.
According to my DB the lowest price is $16.99 for SONOFF D1 Smart Dimmer Switch (in Banggood). You can create price alert here, copy coupons and view price history. Best price?
Latest coupon and price update (UTC):
Banggood ($16.99)
Banggood warehouse is located in China. I’m afraid that there’s no working coupon in Banggood. Is it a reasonable price for this item? From what I can see this price is higher than average price ($9.62).
Link: Banggood product page
Price: $16.99
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/i1u
Old coupons (from the oldest to the newest): elec, BGTT10, bGhOnEy10, BGSMOFF, BGSONOFF15, BGELIG0920, 13f142, BgcLuBofErtA10, BGSONOFFD1, BGSONOFF16, BGAFF10OFF, BGSTHO15, BGCuponation10, BGHONEY2110, BGSHOM15, 6GSGIT, ADMITAD2212, BGSmart6, BGe8f407, MVC8SPRING, 2203ActiesNL, BG15Sonoff
(you can check these old coupons out, but these probably won’t work)
Aliexpress
There’re some similar (or not) items on Aliexpress
You can use link bot to get even lower price:
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 D1 Smart Dimmer Switch. The lowest price was: $9.34.
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)).