This was deliberately left unspecified in the Secret History. We came up with quite a few ideas, but decided it was best left for the GM to choose. You could pick one of these ideas, or just use these as rumours to hide the REAL facts.
Goldmining (details from Alison and Ian)
When the original settlers moved into the area, one of the signs that they had reached a promised land was placer deposits of rune metal in the streambeds. Over the centuries, these have been exhausted. The deposit near Goldbreath was one of the last, and it was panning out. It was winnowed by using wind to separate the heavier gold particles from the dirt. This is an old method where there is a lack of water for washing the silt.
When the Orlanthi settled New Pavis and Garhound, the Goldbreath patriarch of the day made discreet inquiries about their air powers. Eager to have good relations with these powerful neighbours, the Orlanthi priests gave valuable advice on better wind tunnel designs, and loaned sylphs. This enabled the Goldbreath family to reopen the old workings, and resift their spoil heap. Thus, the Orlanthi were able to trade their knowledge for grain while their farmland was still being broken to the plough. Vega would have more respect for the Orlanthi and Vingans because of these links.
Barley Beer
Idea abandoned, when Ian pointed out: "Whatever the wealth of the Goldbreath clan is, it can't be based on a monopoly of the brewing of beer. Beer is going to be brewed in every village, hamlet, and large household. It is one of the major sources of calories for the Sun Domers as well as being about the only really safe beverage. (Don't drink the water!) Any attempt to set up such a restrictive monopoly on a basic foodstuff like beer would have led to a revolt."
Whisky (suggested by Ian as an alternative)
"This is a luxury good, not a basic. It is going to be hard to produce, because the SunDomers are limited in fuel, not barley. You will need a lot of cow-chips to run a still. Any activity that consumes that much of a vital resource like fuel in order to produce a luxury good like whiskey is a good target for special taxation or a monopoly."
I then added: " You don't need 'fuel' (dung or whatever) to provide the heat source for distilling. You need the sun, and the magical equivalent of a lens."
Magically-enhanced brandy: my idea.
I liked the whisky idea, but I think of it as coming from misty highlands with peat bogs, not semi-desert.
So I took another look at the map and description of Goldbreath village, p123 of Sun County. Amazingly fertile place for the location, with a nice variety of crops. Which include grapes and herbs... brandy, not whisky. Keeping my idea of using the sun for distilling. And adding an idea from a Glayva bottle: secret herbs to add flavour. You see, back when Genert was getting kicked, there was a plant spirit living here. Chaos came along, and she did the spiritual equivalent of screaming and burying her head in the ground. When she came up for a look round, the place was awfully dead. In a fit of guilt, she promised to spend the rest of her existence helping in the fight against Chaos. Of course, she was a bit limited in what she could do. It's thanks to her that the place is as fertile as it is, but her special contribution is the berries of her "host" plant. When steeped in strong brandy, they give it extra powers to fight chaos: drinking it, or pouring it into a wound, helps fight infection and disease. It also tastes wonderful! Unfortunately the berries have to be really fresh (not more than 10 minutes after picking), so export is not an option. Jovian's idea at the start of his military career was to down a bottle of the stuff before entering any combat, just in case.
An oil well. I take the blame for this one.
No, we don't take this idea too seriously. What would Sun County do with an oil well? Oil's caused by buried dead plants: wasn't there some Garden destroyed around here a while back? Better stop this before I convince myself: it makes far too much sense!
This little idea was almost certainly to blame for MOB's insertion of Dallas jokes into every aspect of the History.