I don't understand why everyone struggles so much with demand. You can't form a price without that.
Just take good old Maslow's hierarchy of needs. First and foremost, dwarfs need food and booze, so there's a constant demand for that. The price for food is formed by comparing the amount of plump helmets to the amount of hungry dwarfs.
If dwarfs are well-fed and drunk, they want a nice bed and several layers of solid rock over their head. Count the dwarfs sleeping in the mud, subtract the number of beds, there is your demand.
Moving on, there is the need for personal comfort and relationships. That's pretty non-material, so I'll combine it with the next step: self-esteem, achievement and respect. Satiated and cave-adapted dwarfs have a demand for fancy clothes, a barrel of sunshine, maybe a cabinet in their bedroom, admiring a nice door, or they want a pet. This will be a bit more fleshed out after the personality rewrite, but you can see the intentions. (Which reminds me, we'll probably need object deterioration before the economy rewrite. There won't be a lot of constant demand for bracelets if they never break.)
And finally, we have the need for self-actualization, which will only be fulfilled for a few individuals. Maybe a dwarf wants to become a musician, or tame a dragon, or be elected mayor, or engrave the king's bedroom, etc etc. Again, that's not very material, and the personality rewrite will introduce more of these personal goals.
So, yeah, personal needs create demand on the fortress level. The old economy of 40d was working like that, by the way. How does the global economy work then?
It's not much different, actually. The nations have rulers, who want to be happy like everyone else. They try to accomplish that by staying the ruler as long as they can. That works best if your nation's population is happy and protected. Therefore, the national economy is just as well influenced by demands of individuals.
An example: imagine a dragon attack on a human town. Most of the houses burn down, but most of the inhabitants survive, homeless and hungry. Since the town is destroyed, they move to different cities of the civilization. Instantly, there is a demand for housing and food, so to increase the supply of blocks, fish and crops, more citizens become masons or farmers.
Of course, the game can't track the thoughts and wishes of every individual and calculate prices based on that. But based on the overall wealth of a nation, the profession ratios, locations and availability of certain resources, mortality rate, political situation and the baron's love for ballista parts; we should be able to get some fairly accurate economic predictions.
And now you know that each time you read "She has admired a fine door lately", the price for doors just dropped a little.