Xiaomi Aqara temperature and humidity sensor.

* Under $10.

* Works with zigbee.

* Works out of the box with open source zigbee hubs. I personally have conbee II/deconz.

And the whole setup of these open source zigbee solutions usually just consists of plugging in the USB device and starting a docker container. It's not not the tedious open source setup as it often is with open source projects.

As a bonus you can use different zigbee devices from different manufacturers all together.