Given that the target market is "Apple customers", I don't think $25/year/device is much of a burden.

Regardless, though, I don't like the idea of a device that's designed to be landfilled every year; replaceable batteries are the right way to go here.

$100/4 isn't going to be a burden for most apple customers if they're just putting them on their keys, wallets, handbags, etc. But the endgame for these sorts of devices is longer lifetimes and slapping them on everything you might conceivable care about losing, which will only become practical once they hit prices below $1. Maybe something based on semi-passive RFID?

For something that I hopefully won't really need on a regular basis, I'd hesitate to buy something that was essentially a $25 annual subscription.