Gold is probably also not valuable enough on a per volume or per weight basis to offset the transport costs. We'd need something much more valuable, although t

Gold is probably also not valuable enough on a per volume or per weight basis to offset the transport costs. We'd need something much more valuable, although the usual go-to, diamonds, also wouldn't work since it's a highly controlled market. At that point you're in exotic materials territory. Maybe a natural source of Californium which on Earth is only produced in reactors or particle accelerators, and actually has industrial uses? Even if its price dropped substantially, we'd probably find new uses for the new supply and the stuff costs millions of USD per gram.