Here are a few estimates of damage from relevant historical hurricanes had they occurred in 2004. I’d guess, and it is nothing more than a guess, that Katrina will exceed the amounts of Betsy, Camille and Hugo but not Andrew.

1965 Betsy $18 billion
1969 Camille $19 billion
1989 Hugo $16 billion
1992 Andrew $66 billion

For methods, see this paper:

Pielke, Jr., R. A., and C. W. Landsea, 1998: Normalized Hurricane Damages in the United States: 1925-95. Weather and Forecasting, American Meteorological Society, Vol. 13, 621-631. (PDF)