Five Lunar Prospectors would still weigh less than a Ford Taurus. Development took less than two years, liberally borrowing technology from earlier mission. Four days after launch, Lunar Prospector will enter orbit around the moon. Soon after that, it'll start collecting data and continue for at least one year. There is much unknown.
Binder notes geologists have studied the Earth for three centuries.
I think we can see where we are by that comparison. We are just beginning to understand the moon as a planetary body. NASA probably won't wait another quarter-century for its next visit.