If you want the most bang for your buck, when requesting items from the dwarf caravan, I have found no better item to ask for, than caged animals.
Not only do you get the meat from said animals, but if you get in quickly before they're slaughtered, you can also shear them, get a free statue (Totem), can get lots of (admittedly) cheap bone products, meat, some thread that cannot become cloth no matter how much you yell and scream and research, and some atom-smashing fodder.
On TOP of that, you get the cages themselves. Usually a mix of lead, copper, zinc, and tin. Admittedly, 1/3 a bar per cage isn't that great, individually, but once you start buying animals en masse, it adds up.
So not only do you get a product that becomes many different products, that can have cost multipliers added in several different ways (Masterwork cloth with a Masterwork dye job in a Masterwork Craft or something, Prepared Food and all that glorious brokenness, bone crafts (I tend to go for Bone Large Gems, myself. I think all Bone stuff goes from Bone being worth nothing to 120/pop. Not much individually, but if you're buthering lots of large animals, it adds up quickly, and takes almost no time to perform. Tossit all in a bin and forget about it. If you're really stingy, throw it in a Masterwork metal bin made of the cage the animal came in.
I bought about 70 animals, large and small, in the last caravan, and started with somewhere less than 350 meat. Halfway thourgh the big animals (cows, reindeer, horses, etc), I'm at 1k meat. The total cost of everything that I wanted out of the caravan was something near 30-40k, including a lot of leather (I bought the leather itself, not the bin AND the leather. such fun. I bought the bins afterwards, but they're still hauling 650 leather by hand at first) and a crapload of bars. I could EASILY make that up in just the meat alone. The extra metal is just a handy bonus. On top of that I probably got at least 50 bits of thread from the wool of the alpacas, llamas, sheep, and goats. Potentially even more. I didn't keep track.
Also potentially fort-saving, if you have little surface metals, and get a caravan with tin and copper cages.
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TL:DR
I love buying wolly animals from dwarf caravans. Or anyone, really.
Yarn cloth, potentially a few bars of weapons-grade metal, food, crafts (Bone, Totem, AND hoof), and the perfect avenue for FUN! to be unleashed upon your fort by your neighborhood necromancer.