Bruce Schneier has a nice post up on his site talking about the cost benefits ratio of terror attacks. Ultimately terror attacks can be done on the cheap while costing us billions in wasted security measures aimed and preventing that same attack next time. Think the TSA for a second, every time some yahoo gets through with a few bucks with of poor explosives in their britches the TSA spend millions on new technologies that allow them to see through your clothing.
The terrorists have won ladies and gentlemen (well they won the second the PATRIOT Act was passed now that I think about it). If you can bleed your opponent dry of resources you really can hold out longer than they can and thus will win the war.