/usr/share at
I have 10 mbit/s both ways. My router's data show that, during the whole October, I downloaded 86 GB and uploaded 83 (most of it probably BitTorrent).
That is allot of data. I was mainly talking about mobile broadband/data, my bad. :P
Oh. I used to have a 2GB "unlimited" 3G plan for ~15US$/mo, and I only remember going over the limit (at which your speed is limited, but you don't pay any extra) once.
Of course, my mobile browsing habits were limited to text websites -- no music streaming, no YouTube or anything like that.