Aliexpress
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (341.27 USD): BGPROBuscape
Coupon (361.34 USD): BGAProjector
(CouponsFromChina.com tip: coupon code has to be applied in shop’s cart 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): TECH8%, PICODICOM, Wholesale20, aff7off, BG3CSUM, bgssaff9, BGaDmITAd, BGGR193, bgchinafest, BGBFAFF2019, BGCITYADS2019, bGpAnDacHeCk, Email5, BGAFF10OFF, bGhOnEy10, BGdragonBoat, BGAFF7OFF, bgshOOp10, BgoFErTaSchInA10, 7bcdf9, BGProjectors
(these coupons are old, so probably won’t work)
Price history and price alert
Here is price history (for all/most countries) for TOUMEI C800 Mini Pico Video Projector.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).