Hi,
I have made very good experience with a JeeLink controller and appropriate TechnoLine sensors. One sensor (temperature and humidity) costs around 17 EUR and the controller about 30 EUR. I placed one sensor in every room and for openHAB 2 you can find a very good bindig here: https://community.openhab.org/t/new-jeelink-openhab2-binding/15018
Kind regards,
André