I have just checked coupons. The lowest price I could get is $14.99 for RBR/C 7.4V 1800mAh 10C 2S Lipo RC Battery (in Banggood). You can create price alert here, copy coupons and view price history. What is best price?
Combination battery voltage: 7.4v, Battery pack technical parameters (specifically can be designed according to customer requirements-voltage / capacity / size / line), Single battery model: 18650Single battery voltage: 3.7v, Combined battery capacity: 1800mAh, Battery combination: two strings in parallel, Single battery capacity: 1800mAh (full capacity), Battery pack model: 18650 7.4v 1800mAh
Latest coupon code price check (UTC time):
Banggood ($14.99)
Banggood warehouse is located in China. I couldn’t find any coupon for this product on latest product update. Is it cheap? Maybe there’re no more promotions for this item, because current price is higher than average (which is equal $8.72)
Link: Banggood product page
Price: $14.99
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BGe227c2, BGToys8, BGMatt2022, BG589b2e
(I don’t think these’re gonna work, but you can try ’em out anyway)
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Price history and price alert
Price history (for all/most countries) for RBR/C 7.4V 1800mAh 10C 2S Lipo RC Battery is here.
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)).