I had a relay set up last month on a new high speed comcast link (19M/6M ... FAST!). It turns out that over half a month I racked up about 900G in traffic. I assume that this came almost exclusively from my tor relay. Just this week I got a call from comcast tech support saying that comcast caps it's usage at 250G a month. They also told me that if I continued to go over this limit that I would be banned from comcast service for the following year.
Just thought I would pass this on to the larger community. evoltech