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. You can create price alert here, copy coupons and view price history. Best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
You can use link bot to get even lower price:
Click here to search by yourself. Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. Hmmm… There is no active coupon in Banggood at this moment.
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!
Additional Banggood coupons: first page and second page
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
(you can check these old coupons out, but these probably won’t work)
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. The lowest price was: $32.42.
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)).