Aliexpress
Click here… to search for POCO C40 4/64GB JR510 on Aliexpress
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in Hong Kong. I found coupon for you. Check this out.
Link: Banggood product page
Coupon (104.00 USD): BG5f0ad8
(CouponsFromChina.com tip: put item in your cart and apply coupon code there 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): BG77f78a, TTBLACK5, BG1fac9d, BGa0f8df, BG30f050, BGb27bbb, BGee7916, BG3650ce, BGf74ce2, BG89b28b, BG3MEGA5, BG6357a8, BGc6f6de, BGdd9ddf, BG1378d7, BGMARCHSALE, BGcae55f, BGfe2c5d, BG0271b6
(there’s a little possibility that old coupon will work)
Price history and price alert
Here is price history (for all/most countries) for POCO C40 4/64GB JR510.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).