Just check coupons out (with price alert and history). Let’s see what best price is ATM.
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest coupon code price check (UTC time):
Weight: 4g, Type: CPU Card, Brand: Aqara, Each Aqara smart door lock supports up to 25 NFC cards, Size: 26mm*42mm*3.5mm, Material: Epoxy
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
1. ($11.82): Cuponation9
2. ($11.95): TTSUMMER8
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 and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BGaef37b, ADMITAD2212, BGSmart6, BGc40e8c, 8Couponsde, MVC8SPRING, 8BGwbg2204, tRaDetRaCKeR, 8wEbgaInS5, pROMo8cuPoN, BGJUNEGSG9
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Do you need price history chart (for all/most countries) for Aqara Smart Door Lock NFC Card? There we go.
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)).