2x AG13 battery (no included).
Price history, coupons and price alert are available on this page. What is 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.
Link: Banggood product page
1. ($2.77): BGAFF10OFF
2. ($2.77): bGhOnEy10
this code has to be applied 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): TTxBG10, aff7off, bgssaff9, AFfbG10, BGELE2, BGTEP2, BG11VIPD, BG2903, BG2020NS, BGVDay10%off
(probably won’t work, but you can try them out)
Price history and price alert
Price history (for all/most countries) for Mini LCD Digital Thermometer Hygrometer is here. The lowest price was: $1.69.
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)).