Just check coupons out (with price alert and history). Best price?
Welding voltage: 4.2V, Welding current: MAX 650 A, Charging voltage: 5 V, Host size: 103x 83x 29 mm, Charging current: 2 A, Charging interface: Type-c, Weight: 302g (single host), Package weight: 407g, USB output voltage: 5 V., Packing size: 143x 10748.3mm, USB output current: MAX 2.4 A
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. There is one coupon right now! Check this out.
Link: Banggood product page
1. ($52.32): BGGines
you should apply the coupon in your 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): 069e30, 13f142, BGBG33911, BGU33911, BGe2c213, BG1696386, BANGGOODSD
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Price history (for all/most countries) for 650A Mini Spot Welder DIY 18650 is here.
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)).