Each sensor costs $39, or you can pick up a three-pack for $99 for people who actually own houses large enough to need that many independently temperature-monitored rooms. (I live in an apartment, so it’s a little hard for me to imagine that kind of luxury.)
To use the Temperature Sensor, you’ll need either the latest-generation Nest Learning Thermostat or the Nest Thermostat E. (It’s not compatible with older first-gen or second-gen Nest thermostats.) You’ll also need the latest version of the Nest app. While the Temperature Sensors don’t offer complete room-by-room control, they do make it easier to control your house’s heating based on other rooms that don’t have a thermostat.