Terrorism as a threat to the US Economy
There is the potential that terrorism could do some real damage to the US economy. In fact just the threat of terrorism has had a serious impact. The threat of a terrorist attach requires us to invest a large amount of resources in to defending the country. These are resources that can't be used to help the economy grow. Of course the worst impact of terrorism on the economy would be if an attack actually damaged important infrastructure. In a lot of cases we don't have a lot of extra capacity when it comes to things like power generation. Losing an important facility would be devastating.
The main reason that terrorism is a threat to the US economy is the high cost of defending the country against a terrorist attack. In recent years there has been a massive increase in the amount of money that gets spent on security because of the threat of a terrorist attack. All of that money has to come from somewhere and for the most part it is a drain on the economy. Higher taxes and higher user fees are the main costs of this security but there are other expenses that may be even worse. The need to employ people to work in the security industry removes them from the labor pool for business and that drives up the cost of hiring employees. All of the extra costs will really be a drag on the economy.
The threat of terrorism also slows down international trade since anything going into and out of the country will need to be inspected. In a lot of cases companies now need to get permission to export certain products or to certain countries. Any restrictions on trade are going to raise prices and slow down the economy. There is also the risk that if we put to many restrictions on products entering the US out of fear that they may be supporting terrorism that other countries will respond by putting similar restrictions on American products. The resulting trade war wouldn't be good for anybody and would have a serious effect on the economy. A country like the United States depends on foreign trade and would really suffer if anything slow that down.
While the threat of terrorism has a serious impact on the US economy an actual terrorist attack could potentially do far more damage. This is especially true if important infrastructure was damaged as the result of the attack. For example if terrorists destroyed a large power plant the result would be devastating since it would send the cost of electricity soaring. There is also a pretty strong likelihood that in the event of a terrorist attack there would be much tighter restrictions on what companies can and can't do. This would obviously cause the economy to slow down. There is also the problem that any damage that gets done would have to be paid for, another drain on the economy.