Being too safe from terrorist attacks can be costly in at least a couple of ways. The most costly is our willy-nilly acceptance of government intrusions on our liberties and privacy in the name of security. For just one example, banks, brokerage houses, insurers and other financial institutions have been turned into state informers who must notify the Treasury Department about "suspicious" transactions. A customer's attempt to maintain financial privacy can be seen and reported as a suspicious transaction. Our financial records can be subpoenaed and examined even if there is no evidence of wrongdoing.
Government officials have always wanted open access to our financial records; the war against terrorism gives them the cover to do so. Here's what might be proof: How about an amendment to the Patriot Act whereby any information gathered under its provisions cannot be used in a court of law unless it can be tied to terrorist activity? I'm guessing that few politicians and law enforcement authorities would agree to such an amendment.
Most vital to the conduct of any war, including a war on terrorism, is a vibrant, flexible economy. There's a possibility that massive volumes of security regulations and massive security expenditures can weaken our economy and thereby threaten national security. Al-Qaeda type terrorism is not our only national security threat either now or in the future. Keep in mind it was our productive capacity that ultimately won the Cold War.