It a simple matter of supply and demand. Uranium benefits are usually high because it's demanded everywhere, but only produced in a handful of cities. I've been doing some rough research on trade routes, but I don't really care to figure out every detail till civ 3 comes out.
The amount of money and trade is affected by three things:
1. Distance (londer distance = higher revenue)
2. Amout of trade already in a city (hence superhighways enhances trade routes since it increases organic trade)
3. Supply and Demand (high demand + low supply = big revenues)
There are several oddities:
1. Certain resources are only in demand in certian times. Wool and hides are major cash routes in the early game. Oil and Uranuim show up later as major cash resources.
2. Demand is affected by trade. Some resources -dye and coal in particular- may be in large supply but low demand in the late game. However, if you trade these in cities that can trade them, global demand increases. In one game (in 1900) I had fifty cities supplying dye but only 8 demanding. I started trading and before (1930) I had 80 cities demanding it but only 20 suppying it. Likewise profits rose. I might make 200 on a carvan in 1900 but I was making 900 a caravan in 1930.
3. Some resources are always in supply. As you make one trade route in two cities effectively canceling that demand and supply another supply will pop up somewhere. This goes for all major trade items: Dye, Silver, Coal, and Uranium.
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