Haven't really measured that. Setting up you'll eat up like 350 GB downloading everything you need. Day to day running is a lot less.
I've attached my network traffic graph for the last hour. I can't really tell how often its being transferred but I'm guessing per second, and using that as an estimate, its about 70 GB/month which really isn't bad at all. (30 kb/sec * 60 sec/min * 60 min/hour * 24/hour/day * 30 day/month).