Average box office per movie: total chaos
Update: Adam found this great chart, showing box office gross adjusted for inflation -- the field peaked in 1939, with Gone the With the Wind in top place, with an adjusted gross of $450.5 million. The only movie from later than 1990 in the top ten is Titanic.
Update 2: Adam sez, "The figure listed for Gone with the Wind is adjusted to 1977 dollars...when adjusted to 2007, it's the much more impressive $1,329,453,600."