How about a thermistor embedded right under the fabric in the chair? Room temperature probably won't ever exceed, say 76 F. in an office, and when somebody plunks down in their chair, the thermocouple will probably read at least 90 F after, say, 2 minutes. Bring the thermistor wires to the back of the chair to hook to a Nano and a cheap little RF module. That might get your cost down to $5.00. Of course, be prepared for the smart ass who sets his warm coffee cup in the chair as he leaves the desk. Maybe multiple thermistors -- one under each cheek.