As far as wifi data usage, here are some stats from my UniFi controller:
My sense has been connected for the past 3 days 18 hours during which it sent up 428MB of data and received 82.4MB of data. Dividing that out across the time period, that works out to be roughly 1.3KB of data sent to the Sense cloud server per second.
The traffic is a little bursty in that it seems to batch updates up to send until I open the sense app on my phone at which point it gives realtime updates using about 850bytes/second of traffic.
The TI chipset that is used plus the antenna gives the sense a really good signal - my AP is mounted on the ceiling of my two story foyer, and the sense is in the basement. The AP reports it still gets -59dBm of signal from the sense through three floors and several walls.
Hope this helps-