I'd guess at least a few of us have seen that the trading cost for things like certain resources goes up when you learn a relevant technology. For instance, if you learn Chivalry, the trading cost for both horses and iron goes up. In a game I have going right now, I noticed that when I entered the industrial age and traded for Nationalism and Steam Power (Steam Power I would guess the cause), the cost for horses went down. Do we have any sort of comprehensive list for this, or can we make one?