I have no precise data; however, I do have a MacBook Air mid 2012 and I had to buy a Thunderbolt Gigabit adapter. What I can say is that it consumer substantially more than the WiFi chip. Fully charged, I can reach 5+ hours using the WiFi chip; when the Thunderbolt Gigabit adapter, it's enough if I get to 3.5 hours.
Honestly, this was an authentic shock to me, and I blamed the battery at first, thinking it was faulty and needed to be changed; however, using the WiFi, battery life gets back to 5+ hours, so I'm pretty sure this has to do with the Thunderbolt Gigabit adapter.
This is not a scientific test, but on average I use the Mac to surf on the internet, normal stuff. Anyway, I got this decrease in performance without changing my usage pattern.