A rail gun on the moon could fire cargo pallets of valuable metals back to earth for very little money.
Getting there the first time is the expensive part. With the moon’s light gravity and no atmosphere, a solar powered rail gun would become just another basic piece of industrial hardware.
Fire the pallets into Low Earth Orbit to be collected by an autonomous tug for a consolidated delivery package. Have the rail gun send an ablative heat shield made from moon rock to protect the cargo during entry into the earth’s atmosphere.
http://www.youtube.com/watch?v=VBQHtF3WhMw
Earth and Moon Gravity Well Comparison
We can’t even make a rail gun work reliably here on earth. And more to the point what valuable metals? The astronauts didn’t see any valuable metals lying around on the lunar surface. They found rocks and dirt very similar to what you’d find in Hawaii. Nothing that you’d get anything valuable from