My (c)iv screen says I've only exported about 20,000DB worth of stuff to them, and I've easily seized several hundred thousand worth of cloth from them, yet still they persist. Next spring, I'm going to mod my pack animals to carry as much as wagons (not in response to your suggestion - I just happened to come to the same conclusion) - if they then bring five hundred bins of cloth, I'll know something is horribly, horribly wrong.
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I just replayed from my last Spring autosave with donkeys, mules, and camels set to 10 times their original capacity, and they brought a total of 77 bins of cloth, as well as tons of other stuff (including 70 tame animals and 11 vermin). Either their negative profit is completely offset by my Created Wealth (which only recently reached 9 million), or I managed to steal so much from them that their profit amount went so far negative that it wrapped around to positive again.
Additionally, there's actually a small problem in the "cloth is the best value" theory for Elves, especially when they have access to kobold bulbs - a barrel of Gnomeblight is worth 1010 and weighs less than 100 (and in the above test, they brought 78 barrels of Gnomeblight).
It would seem the problem isn't that cloth is the "most valuable" good they have, but that it's the first item they bring when the caravan arrives, most likely due to it coming in bins (to ensure that all of the bins they bring are 100% full) - the first N animals will bring nothing but cloth bins, then one will have a few bins of cloth along with various other items, then all of the others bring random stuff; if they want to bring a ridiculously huge amount of stuff, they end up running out of pack animals before running out of cloth bins.