Netflix on "auto" quality settings, watching "Weeds" for a minute, translates to 60 GB/month (per 1 hr/day). We quickly dialed that back to the middle setting.
YouTube is about one-tenth of that, for regular quality video. HD was triple the regular amount (but it will depend on what you watch). So, 6 GB/month (per hour watched/day) for standard quality. 18 GB/month (per hour watched/day) for High Def. YMMV.