Just check coupons out (with price alert and history). What is best price?
Model: T100, Quantity: 2pcs toothbrush host + 5pcs brush head, , , Package size: Approx. 22×10.5×2.15cm / 8.66×4.13×0.85 in, Brand: Xiaomi Mijia, , Product weight: 46g, Rated power: 0.6W, Color: White/Pink/Blue, Materials: ABS, Rated voltage: 3.7V, Charging time: 4 hours
Aliexpress
There’re some similar (or not) items on Aliexpress
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. I am tired of searching coupons, but I found one for you…
Link: Banggood product page
1. ($25.19): BG10OFFAFF1
apply it in your cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BGwhielabel, 13f142, BGxm7205
(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 2x Xiaomi Mijia T100 Sonic Toothbrush with 5 Heads is here.
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)).