Precise 5 gear test, It is powered by 1 AAA battery (not included), green light means normal, yellow light means the oil still can be used, but you have to pay attention to it, red light means you have to change the brake oil.
You can create price alert here, copy coupons and view price history. What is the 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 coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. You can get coupon down below.
Link: Banggood product page
1. ($12.05): BGRepair63
apply code in your basket (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): BgAuMotor, TTxBG10, AM15prime, AMPrime01, Automo, Automo1, BGCHRISTMAS, BGCARPA, BG20CAR, BGAutos, bGhOnEy10, BG2dfcd7, BGCARELEC, BG32673, 13f142, BGAFF10OFF, BGCuponation10, BGHONEY2110, BG10OFFAFF, BGwhielabel, BGHRepair2, BG10OFFAFF1
(these coupons are old, so probably won’t work)
Price history and price alert
Check out price history chart (for all/most countries) for Brake Fluid Tester Pen. The lowest price was: $2.99.
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)).