Size: 6.5*0.4*1.3 cm, Item Name: 3.7V 520mAh Li-po Battery, Usage: For Wltoys V977 V930 XK K110 RC Helicopter, Flying time: About 11 minutes(Manufacturer rated), Weight: About 13.2g/piece.
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
You can use link bot to get even lower price:
Click here to search by yourself. Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. There is one great coupon!
Link: Banggood product page
1. ($18.93): AFfbG10
apply it in your cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/i1u
Old coupons (from the oldest to the newest): ZXC12, 502C6B, TTxBG10, 2019SPEDM, BGWL10
(there’s a little possibility that old coupon will work)
Price history and price alert
Price history (for all/most countries) for 5x WLtoys V930 V977 XK K110 RC 3.7V 520mAh 30C RC Battery is here. The lowest price was: $17.88.
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)).