You can create price alert here, copy coupons and view price history. What is best price?
BAK – 4 roof prism system, Amplification Factor: 10X
Aliexpress
I could not find it on Aliexpress. Click here to search by yourself.
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Price history and price alert
Check out price history chart (for all/most countries) for Gocomma 10X 42mm Monocular.
Price records source(s): Gearbest
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)).