Sources of a community's wealth in Forgotten Realms
In reply to Ginkgo Bear (msg # 1):
I recommend that you just posit that it's true that the city has a source of wealth, and then work with your players to come up with it. Players will come up with answers that a) interest them and b) are plausible to them, meaning that there's much less effort required to draw the players in and convince them. Also, since the players at least partially came up with the idea, they'll have some incentive to help it make sense within the larger world, or at least not question that it does make sense.
That said:
A new kind of an old mineral. In Neal Stephenson's books there's a form of gold that is identical to normal gold, but is slightly more dense and has mystical properties. Maybe there's a type of silver that is better than usual at harming silver-sensitive creatures, or it's a mineral that it turns out some nuisance monster is allergic to. For instance, Eberron has byeshk, which harms a certain type of monster better than anything else.
Certain magical reagents. In 4th Edition, there are consumable treasure items that can augment certain spell types. These are things like a special sand that gives a bonus to spells that daze or slow, or a kind of exploding nut that augments fire spells. This town has a source for those.
Drugs are always an option. They start as "medicine" of course, until they get really popular. The population of the city doesn't use them, of course, just exports them, which is lucrative, but draws the ire of certain groups. Think of poppy farmers as an example. Or maybe the drug is a necessary aspect of the world economy. Think "Dune."
They export a skill taught only to their people. Maybe it's only taught by a certain house, or it's got a hereditary nature to it, so that you have to marry in to learn it, or maybe only your kids can learn it. It makes a lot of money for the city, but it's also a bit of a curse, as the population leaks away, and is sometimes kidnapped. The Dragonmarked houses of Eberron are bit like this: they're rich because literally only they and their members (usually family members, aside from a lucky few, who they make sure to snatch up) can do what they do.
A certain food. There's a reason why lots of popular real-world foods are named for a specific region or town.
Run these ideas past your players, and then go with their idea, which might not be any one of these, but will be perfectly tailored for your game.