You can't clone them cheaply without the subsidy the data provides. These products are usually cheaper than they should be because the manufacturer knows they can get value out of the data sale.
Not this one, though. It's actually surprisingly expensive for what it is. One could build this with an ESP32 and a couple of jellybean parts to regulate the voltage to what the chip needs and create a voltage divider. Even with a case, it's less than $15 BOM.
You'd have to write the phone app, but that would be as complex as you wanted.
