Aliexpress
Click here… to search for Xiaomi Drone 5100mAh RC Battery on Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (71.70 USD): Mirc20
Coupon (80.67 USD): AFfbG10
(CouponsFromChina.com tip: paste coupon code in your cart and click Apply after clicking “Proceed to checkout”. Click to copy code and open shop)
Old coupons (from the oldest to the newest): BGMiB18
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Here is price history (for all/most countries) for Xiaomi Drone 5100mAh RC Battery.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).