Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (29.50 USD): BGherra
Coupon (31.24 USD): BGAFF10OFF
(CouponsFromChina.com tip: paste coupon code in your cart and click Apply after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BG2199, elec, 9ca9bc, a33514, BGMPJ16, BG219, TS15prime, BGCITYADS2019, AFfbG10, BG2020NS, 0e7f33, BgcLuBofErtA10
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Here is price history (for all/most countries) for 300kg D75mm Neodymium Magnet.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).