What I love the most about DF is that you can produce so many different things and trade with others, but what's the point of trade without proper demand? I present to you a system that could be both easy to implement and would provide basic economy to the game. It will benefit both fortress part of the game - you will have to plan your production accordingly and the adventurer part since you can trade.
Supply and Demand
1. Every settlement should produce certain things, the bigger it is the more things it will produce, and this could be further modified by access to different goods, ingredients, metals, and the general world's demand for goods.
2. Since sites are producing things, they will need things too - if they're producing axes, they will need metals. If they're producing clothes, they will need wool etc. It can be achieved by using a certain number of local settlers - if a 100 dwarves live in a fortress, 25 of them might be producing food, 25 tools, 25 weapons and armors and others will be idlers, nobles, performers, priests, etc. Every dwarf can use 5 iron bars per day, so we have maximum demand for 125 iron bars per day. If 50% of demand is satisfied than the price is 200%, if 75% - 150%, if 100% - 100%, anything else they will buy for 90%, or proportionally less the more iron they have stockpiled.
3. To know how much to produce the game can make a check of needs - if the nearby settlements need 150 axes in total per day, the site will produce and try to sell that amount and will try not to exceed it. Of course we need an element that accounts for economic information not being perfect, so they will most likely produce more or less, depending on their manager's skill or to simplify, randomly between 10% more than really needed and 10% less.
4. The more those needs are satisfied the lower the price, in the same way I explained before. Food can be a different category, since if some creatures are starving, they are willing to pay a lot more to live than someone who needs more iron to produce more and be rich.
5. Food and beverage consumption can be tied to how large creatures are to simplify things, weapons to how much they use and break in battle, tools to how much they produce/mine, and can be random as well - game can make a random site open a new mine, and they will, for example, need 25 picks, and then 5 every month to account for equipment breaking, being lost etc. Again, the same mechanism, and the game needs to keep track of individual equipment that the site has.
6. Materials - game should take into account how much something costs comparable to how well it will perform. Steel and iron picks should therefore be preferred over copper ones if a site can pay for them, if not, it will not choose steel at all, and will only consider iron, if it can't pay for iron, it will consider only copper. The same mechanism can be applied to other things as well. It can be decided by assigning a richness factor to a settlement that will be altered by trade and producing goods. A site that buys a lot and not sells enough will soon be forced to buy cheaper things and then to stop buying altogether, selling its stockpiles in the process as well.
7. Game can compare demand and materials at hand. If a site has 100 copper but the demand is only for 25 copper swords and 75 copper cups, it will produce only 25 swords and 75 cups. If there is demand for 100 swords and 100 cups, it will produce 100 swords since those are more expensive. Again, simple comparison. If there is demand for 100 steel swords but site has no steel, it will try to get it provided that the price of material is lower than the price of end goods.
That's it for now, I will expand it later when I have new ideas. Please comment and tell me what you think!