Cost of Afghan war

With the economic crisis, arguments that the war in Afghanistan is getting too costly are being voiced, just like in Vietnam. Thomas Friedman for example writes:

To now make Afghanistan part of the “war on terrorism” — i.e., another nation-building project — is not crazy. It is just too expensive, when balanced against our needs for nation-building in America, so that we will have the strength to play our broader global role. Hence, my desire to keep our presence in Afghanistan limited.