It's just plain hard to get metals from a caravan.
For my megaproject I've been assigning all my Electrum to 'tip' the quarried out obelisks (and pyramid). In seven dwarf-years of activity, I've only been able to obtain {...does brief calculation...} 15-20 bars of the stuff form the human and dwarven caravans, even after making mid-to-high level requests. And last time I checked I was getting 7-10 wagons from the humans, as well as pack animals.
Noting that I haven't (discovered) proper precursor deposits or specifically requested the components, and until I tap the magmapipe that I know is there I'm holding back on the active metal industry anyway, apart from fulfilling nobility requests. (It's not so much that I don't have the wood, more that I'm humoring the elves and can't currently use superfluous wooden furniture from the early days as fuel.
IRT Rolon: Might try that idea, myself. I've got loads of junk I want to get rid of, and I've been thinking of gifting a few score of my masterworked mechanisms to the elves, or whatever they can handle after I've traded everything worthwhile from them (including the oversized clothing that matches the human/dwarf trading bonuses). I'll try some pure gifting and mix in some 200% profiteering for them. Even the elves (who I know I won't get electrum from). For most of the time I've been getting (and taking advantage of) 1:1 bartering values without complaints, even though I could spare mountains of Kobold and Goblin clothing, rock instruments, mugs and other items of massive availability.