“The Theory of Interstellar Trade” by Nobel Prize winner Paul Krugman is an interesting paper from 1978 on the basic economics of trading between two stars without traveling faster than light. The author describes a situation where Earth might trade with a planet called Trantor. (Asimov fans will get the reference).
His conclusion is that it takes hundreds of years to move products from one planet to the other, so trade looks like a long term bond to the trading partners and that the interest rates will equalize between the two.

http://fermatslibrary.com/s/the-theory-of-interstellar-trade

I have written an unsold story called “The Long Run” about interstellar trading without faster than light travel. I wonder what happened to it?

More about Krugman: http://www.newyorker.com/magazine/2010/03/01/the-deflationist (I love this!)