What you want to check is "Trader Proft" versus the value of selected goods. Both are on the trader's side of the screen. These are merchants, remember? They don't want an even trade - or even close to one. They want a profit.
They're generally ecstatic with the trading if they're making twice the worth of what you're taking. Sometimes, that's pretty much the value I have to shoot for before they'll make the deal. I doubt they'll take anything less than 25% over the traded value.
But maybe I just have a lousy broker.
If you don't have enough stone crafts, mugs, toys, and instruments lying around (mugs build wealth super-fast since each order makes 3 of them), you can always trade food. Prepared meals, especially the lavish ones (roasts), especially from really good cooks, sell for obscene amounts of dwarfbucks. See also cloth trade, because a well-dyed, well-made, well-decorated, well-gem-encrusted pig tail sock with a well-dyed, well-sewn image on it will also fetch a thoroughly ridiculous price. Make it out of cave spider silk to really seal the deal.
Metal industry too hard? Try textiles. With a strong textile industry, you can afford anything, even that masterwork steel armor the caravan keeps bringing you whether you asked for it or not.
You could also encrust bolts with gems. That's outlandishly expensive, too.