Economics is very abstract. The only economic resource is gold, which is magically produced in large quantities by cities, and in lesser quantities by Castles and Monastaries. You can get more gold from your holdings if you build gold-producing improvements, which means you can kinda create an EU-style Center of Trade by building a big-ass harbor and other improvements, but the center-of-trade with massive amounts of trade goods changing hands thing is all in your imagination. In-game your city's gold farm just got bigger.
Taxes are the main way to make cash. If you've got a large country taxing vassals is gonna be your best bet. If you've got a small country you probably don't have many vassals, and the ones you do have are probably pretty poor (it's not like you're gonna give some Duke your richest Barony), you'll be taxing your holdings.
Nick