Homelab-priced sensor is the temp sensor in your server, it's free! Actual servers have a bunch, usually have one at intake, "random old PC" servers can use motherboard temp as rough proxy for environment temp.

Hell, even in DC you can look at temperatures and see in front of which server technican was standing just by those sensors.

Second cheapest would be USB-to-1wire module + some DS18B20 1-sire sensors. Easy hobby job to make. They also come with unique ID which means if you put it in TSDB by that ID it doesn't matter where you plug those sensors.