How much did World War 2 cost in today's dollars?
According to http://www.cwc.lsu.edu/cwc/other/stats/warcost.htm, WW 2 cost $2091.3 billion in 1990's dollars.
According to President Truman's address to Congress in 1948 (in part about the emerging Greek Crisis), he stated that the US had contributed about $341 Billion to World War 2. Assuming he mean 1945 dollars, that works out to be about $4.1 TRILLION in 2009 dollars.
Reasonable estimates are that the US spent about 20% of the total of all countries in WW2. That would mean that the total spending for all countries in WW2 was about $20 Trillion 2009 dollars. As a comparison, the current total global economy is about $58 Trillion, and global defense spending in 2009 is about $1.6 Trillion.
So, roughly speaking, we would have to spend about 2.5 times as much to equal the spending during WW2. However, note that the world economy is significantly larger today than during the 1940s, and not just in dollar amounts. So, actual relative spending in WW2 was more likely in the 8-10 times range than current.