What comes back to Earth?

All proposals for space colonization require substantial amounts of
material to be made on earth and launched to destinations outside
this pesky gravity well. A crucial question, as has been raised
many times before, is what's in it for the earthlings?

I just looked at the Summer Study, and I was surprised to see that
they had "no alternative" to solar power stations.

Energy
http://lifesci3.arc.nasa.gov/SpaceSettlement/75SummerStudy/Chapt4.htm
l#SSPS
Information
communications and earth-observation satellites
knowledge about the rest of the universe
services: laboratory research, movie-making, lawyering,
banking, commercial wheeling-and-dealing, etc.
High-value low-mass products
pharmaceuticals
precious metals
Protection from asteroid impact
(Anything that was on course to cause a catastrophic impact
would be put to use instead.)
Tourism
Military services
(The threat of dropping things from orbit on your enemies
is pretty valuable to the people pushing the buttons.)
Nothing
opportunities for emigration

That last one may need some explanation: If life is better in
space, but decelerating payloads down the gravity well precludes
most categories of material imports, people will pay for their
tickets out. It's not likely to be enough to cover all earthly
costs of the destination, but it can contribute to more than the
cost of the person's transport. "Want to move to L5? Pay 30% of
your air bill in advance, plus a deposit for transport back if you
want to be guaranteed that there will be anything outside the
airlock for you in the event of default on future air bills."

I think the big category is the information. A large fraction of
the GDP of the United States could be transported on a beam of
light. If Wall Street can receive tangible goods from the rest of
the planet and export only numbers, so can L5 -- if we can just make
L5 a better place for stockbrokers to live and work than New York.
And that's a reasonable criterion for the thorough success of a
space colony anyway.