This is an overview of the mechanics governing the buy/sell prices of commodities in Europe. It's a bit complex and there is some math involved. The game uses the term "yield" to refer to the commodities (horses, silver, rum, etc), so I'll use that term here.
First I'll go over the variables that determine the prices then I'll explain the actual mechanics.
1. YieldBoughtTotal
For each type of yield, this value tracks the total amount of that yield that you've bought minus the total amount you've sold throughout the game. It starts off at zero. For example, if you sell 100 silver, the YieldBoughtTotal for silver goes down by 100. If you buy 300 horses, the YieldBoughtTotal for horses goes up by 300.
2. PriceChangeThreshold
For each multiple of this value that YieldBoughtTotal exceeds (positively or negatively) the price has a higher and higher chance of changing. It's stored in Assets\XML\Terrain\CIV4YieldInfos.xml.
