Economic Structures

It doesn't matter whether you are writing a fantasy, science fiction, or contemporary fiction novel, you need to make sure your social economic structure is believable. One of the biggest problems that you see is a self contained society where there is no way for them to produce food. There are three things to keep in mind when creating a civilization or government:

Who is in charge?
How does the government get their money?
What do the people get in return?

A lot of stories are about the evils of the ruler overtaxing their people but there is a purpose to taxes. Taxes are how the rulers pay for things such as an army, roads, or other government services. The denser the society the more control the government needs to have. Governments exist on the tolerance of the people.

When you have a more advanced society you have to remember to cover the basics. You can't develop trade until one person can produce enough food for multiple people. This is the difference between hunters/gatherers and agricultural surplus. The downside of the agriculture is that it has lower nutrition than the hunters/gatherers.

You always get a black market if the government restricts a commodity. The cut also has to be high. This is also how piracy develops. It isn't always the physical worth of the materials but the emotional worth. Consider eBooks. People are willing to pay several dollars for something that is actually cheap. There are always frills that people are willing to pay for. For historical examples think of pearls, tulip bulbs, and even diamonds.

When trying to determine what kind of society you have there are a few questions to put to your characters:
How does my character make a living?
Where does food come from?
What is daily life like?
Where does my character fit economically?