[oarsinsync]

> All this can be done for years on a single battery charge and at a cost of pennies per sensor per day

Assuming pennies and years both (as plural) == 2, you're looking at a minimum of $14 per sensor.

$15-30 feels like the right ballpark, so OP is in the right area.

[kayhi]

from where or are there brands you recommend?

[oarsinsync]

I'm mostly in the zigbee space, rather than bluetooth. Aquara / Xiaomi have a load of cheap sensors available on aliexpress. Get yourself your own zigbee dongle (suggestions at https://www.zigbee2mqtt.io, no affiliation, also available from aliexpress) and you have no vendor lock-in either.