When I was updating this page the price was 7.39 USD for 400mm T-slot T-Miter (in Banggood). You can create price alert here, copy coupons and view price history. Let’s see what best price is ATM.
Latest coupon and price update (UTC):
Banggood (7.39 USD)
Banggood warehouse is located in China. I have no active coupons for you right now. You probably want to know if this is a good price. From what I can see this price is higher than average price (5.64 USD).
Link: Banggood product page
Price: 7.39 USD
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BGMCT01, BGGines, BGTACC18, BG589b2e, BGHTT15, BG20236Tool
(probably won’t work, but you can try them out)
Aliexpress
Click here… to search for 400mm T-slot T-Miter Track on Aliexpress
Here are coupons that you can use.
Price history and price alert
Here is price history (for all/most countries) for 400mm T-slot T-Miter Track.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).