[ohazi]

I wouldn't use an esp32 if your concern is power consumption, as they tend to be pretty power hungry, even when they're only doing BLE. The Nordic nRF52 series is pretty good, and you can find shielded modules for around $10-15 in single unit quantities.

$2 is a bit of a stretch... You can probably get the bare IC for ~$5 if you're willing to do the antenna yourself.