This Wired Magazine article puts the case forward for Helium-3, an isotope that could have commercial applications in the energy industry. The mining of Helium-3, in abundance on the Moon but scarce on Earth, was on of the many objectives for the proposal of a permanent lunar colony. It was estimated that a space shuttle load of Helium-3 can power the United States for a full year.
Looks like the United States face competition too from other space-faring nations like Russia, China and India who are mulling more moon exploration missions too. Space Race II anyone? This race might prove to have more economic benefits than ideological ones.
The Major Reward: Leadership of the post-hydrocarbon industry 50 years from now..