3.7V 455mAh, 36 hours (Lights on), Standby 180 days.
You can create price alert here, copy coupons and view price history. Best price?
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 price update (UTC time):
Banggood
Banggood warehouse is located in China. There is one coupon right now! Check this out.
Link: Banggood product page
1. ($56.71): 935bc1
coupon code has to be applied in shop’s cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Additional Banggood coupons: first page and second page
Old coupons (from the oldest to the newest): a9e2f1, AlibuySH50, 0a4f61, Alibuy91224, LSTR93
(these coupons are old, so probably won’t work)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. China – Xiaomi Youpin Smart4u Bicycle Helmet Black ($-3)
Price history and price alert
Check out price history chart (for all/most countries) for XIAOMI Smart4u SH50 Cycling Helmet White. The lowest price was: $35.99.
Price records source(s): Gearbest, 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)).