Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. I found coupon for you. Check this out.
Link: Banggood product page
Coupon (169.48 USD): BGHONEYSP
(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): BG6c2dad, BGMARCHSALE, BGAUGUST8, BGCAPITALOS8, BGCOUPERT8, BGBFDAY8, BGNEWYEAR8
(system checks these coupons on every price check (like once per 24 hours), but you can check them out anyway)
Price history and price alert
Price history (for all/most countries) for Creality 3D 310*320*4mm Heated Bed with Back Support Ultrabase Glass is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).