Battery Life: 60h(always bright) or 90h(flashlight), Light Color: Red.
Basically copy coupon, view price history or create e-mail price alert. 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.
Link: Banggood product page
1. ($9.16): bGhOnEy10
2. ($9.16): BGdragonBoat
3. ($9.16): BGAFF10OFF
4. ($9.16): BgcLuBofErtA10
just apply this code in your 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): bgsport, 502C6B, AFfbG10, BGVDay10%off, BGSS10%SW, bgshOOp10, BgoFErTaSchInA10
(system checks these coupons on every price check (like once per 24 hours), but you can check them out anyway)
Price history and price alert
Here is price history (for all/most countries) for ROCKBROS Bicycle Tail Light. The lowest price was: $8.24.
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)).