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.
"50% is pretty bad."
Spiderman 2 & SW:EpIII dropped about 50% and they're considered successful movies. Well known titles seem to do this. DVC did drop more (a good thing IMO) but it's nowhere near the bottom of the barrel in drops (at least according to BoxOfficeMojo charts). I'm not sure how you're getting a 63% drop if not for the holiday weekend... DVC's 56-57% drop was for the 3 day weekend, if compared from last weeks (3days) to this weeks 4 day weekend the drop is 44%.
I agree the movie industry would make more money if they put out a quality product but they don't seem to care about that. Perhaps they're counting on the international box office to lap up their drek.