There are three elements to the Dominions economy: gold, resources, and magic gems. Blood slaves are an additional economic element of prime importance to blood nations.
Gold is used to buy national troops and commanders and to build temples, labs, and forts. All provinces produce 1 income per turn per 100 population; this is modified by scales. Some sites also increase a province's income. You collect income from any friendly province which has an unbroken route to a friendly fort.
Most nations will do the bulk of their fighting with troops and mages bought with gold. These troops have an upkeep equal to 1/15 of their gold cost per month; sacreds instead cost 1/30 of their gold cost per month.
While gold is a nation-wide asset, resources are tracked province by province. Each province produces a certain number of resources; forests and mountains produce extra. A province without a fort can only make use of half of its total resource tally to recruit indies. Forted provinces can use the province's entire resource count, plus a percentage of the resources from each adjacent province equal to the fort's admin value.
Resources are increased dramatically by production scales, and are required to recruit troops with heavy armor. Nations such as LA Man with excellent heavily-armored troops often take production scales, while nations whose troops cost more gold than resources like EA Mictlan often take sloth.
Gems are used to power powerful spells and forge magic items; see the page on magic. They are often as important as gold for late-game nations, since they are required to summon powerful mages and units and to cast crucial rituals.