All that will also affect what resources your civilization has available. For example, a conquest might give your civilization access to a new source of iron or to a trade route with a distant civilization, affecting the prices of the goods you buy from (or sell to) the caravans. If an enemy civilization razes one of your civilization's settlements and kills a bunch of skilled weaponsmiths, the quality of any further weapons you import might be noticeably reduced - and the refugees fleeing from the destruction will look for a new home and might end up at your own fortress.
So the entire World economy could be computed by what Resources a Civ has access to, and the professions of the Population of that Civ. That would be much easier to keep track of than actually creating and tracking items at the world level.
Thus if you start a new fortress, and determine your parent civilization is lacking in good steel armors, then you can focus on training up high-quality Armorsmiths and producing steel plate armor. For the initial several years you'd receive extremely high prices for your exported steel armor. As your civilization 'absorbs' these goods though, the price would go down as the game computes the contribution that your Dwarves are having to your civilization.
Conversely, maybe you somehow find out where that goblin civ you're at war with keeps the majority of their smiths. So you go attack the site and kill them all, severely hampering their arms production. In subsequent battles, you notice the quality and quantity of their equipment has been greatly reduced.
It would also mean that you would no longer receive 'requests' from the liason and Human merchants, and instead would just receive and update on that Civilization's current prices.