Just check coupons out (with price alert and history). You probably want to know what is the best price?
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 and price update (UTC):
Length: 50mm, Shaft diameter: 3.175mm, Max voltage: <13V, Continue current: 60A, Watts: 900W, Max RPM: 50000, Shaft length: 15mm, Max Amps: 69A, Dimension: 50 * 35 * 34mm / 2.0 * 1.4 * 1.3in (without cables), Diameter: 36mm, Power supply plugs: T plug male, Item name: waterproof ESC, Motor plugs: 4mm banana plug female, Plug type: 4mm golden banana plug, BEC output: 5.8V / 3A, Burst current: 320A, KV(RPM/Volt): 3900KV, Battery: 2-3S Li-Po / 4-9S Ni-Mh Ni-Cd
Banggood
Banggood warehouse is located in China. I couldn’t find any coupon for this product on latest product update.
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): BG95RR98, BG95YY98, AFfbG10, BGRCPART, 502C6B, BGBFAFF2019, BGCITYADS2019, BG129SUR, BGVDay10%off, BG10RCPARTS, BG10PARTS, BGAFF10OFF, bGhOnEy10, BGdragonBoat, BGSS10%SW, bgshOOp10, BGRC902, BgoFErTaSchInA10, BgcLuBofErtA10, BGCuponation10, BGHONEY2110, Parts8, BGRCParts8
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Check out price history chart (for all/most countries) for Surpass Hobby Waterproof 3650 3900KV RC Car Motor With 60A ESC.
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)).