More ideas on economy from Alex (and I also some thoughts of my own ):
Instead of setting prices manually, make them dependant on quantity of that product in store.
If there are very few of iron, for instance - price gets up. More then 5 units - gets down.
Each town would have one (or several) resources that it produces (they regenerate every day, from a few for foodstuffs, to 1-2 to luxury goods) or consume - again, every day, depending on the product.
Best to make towns specialize an producing various goods, and set their required stuff accordingly.
Food is required for every town. Some towns will grow it (and would buy tools). Most of others will consume it on daily basis. When on of towns will start to get out of food, a caravan will be generated at one of nearby 'agrarian' (or fishing/mean producing) towns that will take food from there and carry it to the starving town (the price will be high).
A few other examples:
Town that produce tools will require a lot of iron.
Towns that produce mean and fish will require salt.
Town that produce oil and vine will require pottery (all other towns too, but much less... like 1 a week, to compensate for natural loss, heh).
Capitals will serve as hubs for traders, since they will consume a vast amount of goods, including luxury ones.
And while it should be possible to trade with enemy cities, they should charge extra/low (when you buy/sell respectively) prices depending on your 'criminal' status in them.
Not that hard to make (it might get some time to think out and balance, though), but will make trading much more enjoyable.
Spare Rangers has similar system, and it's considered on of the best games of all time here in Russia
.
My suggestions for extra ware types:
Coal.
Bread (or similar). (I suggest making a chain of town that produce grain, then mill it to flour, which then gets baked into a bread... I hope it's not ahistorical somehow).
Clothes.
Luxury clothes.
Sunflower seeds, or whatever is used to produce oil. (Can be used as a basic foodstuff too).