You can create price alert here, copy coupons and view price history. You probably want to know what is the best price?
AIE 4th Generation AI Engine, RAM 8GB LPDDR4x 2133MHz, OS MIUI 10, Resolution 2340*1080 Pixel, CPU Qualcomm Snapdragon 855 Octa Core (1x 2.84GHz + 3x 2.42GHz + 4x 1.80GHz), ROM 128GB UFS2.1, GPU Adreno 640, Game Turbo Yes, Display Size 6.39 Inch AMOLED, MI Turbo Yes
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):
Banggood
Banggood warehouse is located in China. I’m afraid that there’s no working coupon in Banggood.
Link: Banggood product page
Price: $-3
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): 3BGM98, 6OFFXM, BGaDmITAd, 7bcdf9, od7off, BGBFAFF2019, BGCITYADS2019
(I don’t think these’re gonna work, but you can try ’em out anyway)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. China – Xiaomi Mi9 8/128GB Grey ($-3)
Price history and price alert
Price history (for all/most countries) for Xiaomi Mi9 8/128GB Blue is here.
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)).