They're saying MoS would make in 3 days roughly what BB made in 5 days. Not ideal, but not as horrible as you made it sound, either.
Still, I think they're lowballing at the moment since they haven't seen what WB's primary marketing push will look like yet. When we get close to release, assuming the next trailer is awesome and action-packed and the marketing has kicked into overdrive, I think they'll raise their predictions quite a bit. Right now, I'm predicting it will have closer to an $85m opening weekend, yielding a $260m domestic take and a $500m-$600m worldwide total.