You can create price alert here, copy coupons and view price history. Let’s see what best price is ATM.
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest coupon code price check (UTC time):
diameter 55 mm, The spindle radial runout about 0.01 to 0.03, Spindle net weight about 1.4KG, Insulation dielectric strength 400 V, Diameter 55 mm, Speed 12000 r/min, Insulation resistance > 2 ohms, Power 500 W, Torque 0.45 (N.m), Distance 0.6 (N.m), Working voltage 48 V DC, Insulation resistance > 2 ohms, Insulation dielectric strength 400 V
Banggood
Banggood warehouse is located in China. There is one coupon right now! Check this out.
Link: Banggood product page
1. ($118.78): BGherra
coupon code has to be applied in shop’s cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BGCITYADS2019, AFfbG10, BG2020NS, bGhOnEy10, BGAFF10OFF, BgcLuBofErtA10
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Check out price history chart (for all/most countries) for DC 48V 500W ER11 Brushless Spindle Motor.
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)).