In that case, it will probably be something bigger and more expensive like five or ten of those new airbus planes which flies into the next group of buildings...
While I do think they'd like to do something bigger, the terrorists don't seem to be interested in things that are really expensive. They've shown a habit of using ready-made materials to carry out their attacks like cars, planes, IEDs, cell phone detonators, suicide belts -- things that are relatively cheap for the amount of damage done.
The point is that whatever they'd like to do, you can be sure it could be done with a lot less money than what can be had with $3.50 gas.