Virtually all films fall in box office by at least 60 percent after a blockbuster opening weekend. Da Vinci has done much better than expectations in the US; and its foreign box office is about twice that of the US, breaking records in places like Japan, Germany, Italy and many smaller countries.
We should also mention that X-Men III came out the second weekend which would have overshadowed Da Code.
Er, no. Typical falloffs are in the 30%-40% range. Anything about 50% falloff is a pretty good sign of a dog suffering the effects of bad word-of-mouth.