Aliexpress
Click here… to search for Audew 0-25.4mm LCD Digital Vehicle Tyre Tread Depth Gauge on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. There is one coupon available! Pretty cool, huh?
Link: Banggood product page
Coupon (7.35 USD): BGSCM11
(CouponsFromChina.com tip: put item in your cart and apply coupon code there after clicking “Proceed to checkout”. Click to copy code and open shop)
Old coupons (from the oldest to the newest): BGdcbfa8, Cuponation9, BGSCM75, BG9449bf
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Price history (for all/most countries) for Audew 0-25.4mm LCD Digital Vehicle Tyre Tread Depth Gauge is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).