I think this may be the lowest price ever for the second-generation Ecobee with one extra sensor:
Or about $260 with three extra sensors.
The Ecobee is a well-engineered Smart thermostat with its own additional sensors to allow for different types of zone control.
This is the model which is HomeKit – compatible, and works well with SmartThings using @yvesracine 's code.
He has six or seven different smart apps for different purposes. See the listings in the community-created wiki: