Ditto... a 50% drop for a movie that opened big is not unusual and it should end its run with more than $200mil in domestic theatrical sales alone. In the movie biz, that spells success.
50% is pretty bad. The fact that several movies last year dropped 50% doesn't mean it isn't lousy; last year was the worst year in history for the motion picture industry. Given population growth, economic growth, inflation, and market growth, the motion-picture industry, including VHS/DVD, should have been 30% higher than it was, which is partly why stocks for several movie-related companies have collapsed.
But the Da Vinci Code didn't drop 50%. It dropped 57%. It would have dropped 63%, if it weren't for the holiday weekend. That puts it in the territory of being one of the 25 worst drops in the last 25 years, out of nearly 2000 major releases.