Aliexpress
Click here… to search for 11.1V 550mAh 80C LiPo RC Battery with JST XT30 on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (13.56 USD): bGhOnEy10
Coupon (13.56 USD): BGdragonBoat
Coupon (13.56 USD): BGAFF10OFF
Coupon (13.56 USD): BgcLuBofErtA10
(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): Charger12, BGCITYADS2019, AFfbG10, BGVDay10%off, BG10BC, BGBC1, BGRCBC7, BGSS10%SW, bgshOOp10, BGRC902, BgoFErTaSchInA10
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Check out price history chart (for all/most countries) for 11.1V 550mAh 80C LiPo RC Battery with JST XT30.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).