Amazon offers the Ecobee3 Room Sensor 2-Pack, model no. EB-RSE3PK2-01, for $54.89 with free shipping. (Walmart charges a few cents more.) That's the lowest total price we could find by $6, although we saw them for $2 less in November. These sensors are designed to work with the Ecobee3 thermostat and each measures temperature and occupancy.

Update: The price has increased to $55.33.