[Phantasm]

The basics of an economy on a macro scale is roughly as follows: Produce an excess of one or more products, sell elsewhere for enough money to buy what your region is lacking, and keep in transit/storage for as short a time as possible.

For instance, one nation may have control of the most fertile farmland in the area. This likely means they produce an excess of grain, which they export to neighboring kingdoms in exchange for raw metals. A mountainous region would likely have more metals, coal, stone, and possibly timber than they have farmland, which would provide exports there with which to buy the grain. Another nation could be the prime source of a luxury item - a precious or semi-precious stone (f'rex: emeralds or lapis lazuli), incense, silk, etc - while needing to import a metal (iron, tin, etc.) and/or grain.

Like I said, this is the simple form. Figure out what your nation imports and exports, and match them to other nations. You don't need to work out "$2M GNP" for games; figuring "we have a shipment of cast iron ingots headed for the trade town up river where we'll buy food" is lots easier.