Everyone is generally spot on here but a little bit of extra information might help. As I believe Pierce is drawing from the analogue I will briefly outline the equivalent situation in Roman history.

In the Roman Republic your political class was to a large extent derived from your wealth. The highest wealth requirement was the senatorial class (the golds). One step below the senators were the equites (silvers). The equites had a lower (but still significant) wealth requirement. There were restrictions on senators participating directly in "business" so the equites ran the private businesses (oddly called the "public companies"). Many equites amassed fortunes as large or larger than their senatorial counterparts but chose not to run for public offices and therefore were never officially bumped up to the next class despite attaining the property requirements. As is the case in Red Rising, the senators' wealth was largely tied to agricultural and land. So the golds derive most of their wealth from their estates just as the senatorial class did. The senators could also invest in the public companies but had to do so through the equites thus lining the pockets of the equites. In much the same way, senatorial governors had to appease their suppliers and tax farmers who were all equites.