Aliexpress
Click here… to search for Xiaomi CoClean TK Negative Ion Tower on Aliexpress
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. You can get coupon down below.
Link: Banggood product page
Coupon (563.99 USD): 7bcdf9
(CouponsFromChina.com tip: this code has to be applied in your cart after clicking “Proceed to checkout”. Click to copy code and open shop)
Old coupons (from the oldest to the newest): TTxBG10, bea3e6, vipupgrade8, ML15%Pro, BGHC20, 502C6B, bgshOOp10, BGHB20, BgoFErTaSchInA10, bGhOnEy10, BGSD52469, BgcLuBofErtA10, BGAFF10OFF
(these coupons are old, so probably won’t work)
Price history and price alert
Price history (for all/most countries) for Xiaomi CoClean TK Negative Ion Tower is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).