http://costofwar.com/

Interesting.

Financial Cost of WW1
$196.5 billion (adjusted for 1990 dollar values)

Financial Cost of WW2
$2,091.3 billion (adjusted for 1990 dollar values)

Financial Cost of WW3
Costs are started when 911 occurred. If WW3 was to happen, the start date for the series of events would be 2001.