Running a insurgent campaign is expensive because you have to pay salaries. Running a terrorist organization is very inexpensive because people volunteer.
Peter Bergen, Al Qaida Expert on NPR
However this expert distinguishes between terrorist groups like AlQaida and insurgent groups like Hamas or The Party of Dog or those political Irish murderers the IRA. I would consider all these groups terrorists. However I do admit the distinction between the mad bombers and the more bureaucratic terror groups that also steal, rape, kneecap, shakedown, assassinate or simply murder. It the religious or political justification of the crimes confirms the terrorists as evil not their organization structure.