Wattage: 2W, Voltage: 3.7V, Input: 5V / 1A, Built-in 700mAh Li-ion battery.
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 coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. We have found one coupon for this product!
Link: Banggood product page
1. ($34.64): BG10OFFAFF1
put item in your cart and apply coupon code there after clicking “Proceed to checkout”. Click to copy code and open shop.
Additional Banggood coupons: first page and second page
Old coupons (from the oldest to the newest): vipupgrade8, HP920, 21e5ac, 578e5d, 6ba6cc, 2113e2, TTxBG10, 2019SPEDM, BGTB199, BGss8%hl, bgssaff9, AFfbG10, 502C6B, BGc7b432, BGMIBEI, BGBEI21N, FBCOUPON1, BGAFF10OFF, bGhOnEy10, BGIFG36, BGFIT285, BGFURH48, BHJULY262, bgshOOp10, BGHome15%, BGHOMEA15%, BGZASDE05, BgcLuBofErtA10, BGZKSDK05, BGCZCNDRBEI05, BGCuponation10, 13f142, BGAP0903, BGHONEY2110, BG10OFFAFF, BGwhielabel, BGIS2604, BG1002
(you can check these old coupons out, but these probably won’t work)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – Xiaomi Mijia BET-C01 Sonic Toothbrush [EU] ($-3)
Price history and price alert
Price history (for all/most countries) for Xiaomi Mijia BET-C01 Sonic Toothbrush is here. The lowest price was: $12.99.
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)).