I've noticed this too, I assumed it was a hidden offer/demand agreement of sorts. Didn't know about decorations, but I think decoration are not the whole story, I remember trading clothing products (socks, shoes, capes, etc) and crafts, usually my trade clothes are either low-quality non-decorated rejects from my own production or foreign clothes that I sew images on to make them count as my fort's. The same with random crafts, either redecorated crafts from the previous caravan or mass-produced non-decorated from my own production.
Something I noticed is that the price can go both ways, up or down, so be very careful because sometimes those sock that you think are valued in 100 for the traders is only worth 50 or whatever bellow its "base" price.
I usually go throw my stocks on the trade screen and only sell the ones that have extra profit or base profit, anything below is a waste of value since most likely the next caravan will pay base price or a premium for it. Of course in late years when you're swimming in master-works it gets less useful.
I've noticed that the preferences seem to be category wide, e.g. they pay socks at 200% or bracelets at 50% and so on, I think I remember seeing some oddities though, like some socks being worth 200% while a some others be worth less, that could be due to decorations, most likely because of the trade goods I usually use, as described before, I didn't notice a big difference regarding decorations except of some random oddities, usually the decorated items that I have for sale are former caravan's white-washed ones. I even recall thinking it was probably a difference on the dyes used to colour the cloth fabric the clothing items were made of, I guess that counts as a decoration too.
By the way can't this be seen on the "c"ivs screen? notice that you have to check before the caravan enters your map.