The Sept 11 attack price tag seems to be in the US$200-500k range. A new jet airliner is [link|http://www.boeing.com/commercial/prices/|going to run you] US$80+ million, depending on the options package (air is included, chrome, and sound system extra). A used aircraft could likely be aquired for a fraction of that price, but still an order or two of magnitude more than the cost of the attacks.

Crime pays. Particularly when you're using it to commit more crime.