In one of my recent forts I played, I noticed that some decorated items that were identical in material, material quality, and assigned value were priced differently by the caravan merchant I was trading them to. I was trading masterwork cave spider silk bags with exceptional cave spider silk sewn images, a byproduct of my attempts to make masterwork bags made from masterwork cloth decorated with masterwork images made from masterwork cloth, and many of the bags were valued differently by the merchant.
All of the bags were exactly the same except for the design of the image, so the 'trade agreement' I made with the dwarven diplomat the previous year shouldn't have any effect on the price of the individual bags made from the same materials. Some of the bags were valued higher than others, some much higher, while one was even valued lower than the bags stated absolute value. Bags with similar decorations seemed to be valued at similar levels, but I didn't really have a large enough sample to confirm it. I had no way of accessing the merchant dwarf's likes or dislikes list, so I don't know if that is in fact what was causing the difference in prices.
It makes me very curious. I've never noticed this until I started experimenting with decorations. Has anyone else seen this behavior?
Also, when I decorate something, it seems that I'm actually losing value on the items combined in most cases. The total value of the uncombined items is often higher that the value of the final decorated product, except when the final item's decorations are masterwork. Is this intended?