Number of Americans who died in 9/11: 3,000
Number of Americans who have died of AIDS: 500,000
[added 2011-07]
Amount we will spend trying to prevent another 9/11: hard to say how much when you add in all the new weapons systems, all the health care costs for veterans, replacement costs for weapons used, etc., etc., etc., etc.: — certainly over $2 trillion. Afghanistan alone is approaching $1 trillion dollars, for which we will have the gratifying experience of seeing it swirl down a sinkhole a few short years after we inevitably leave.
Amount spent to stop people dying of AIDS: $150 billion