What about a fabricate spell? (to make a gold statue using the gold in the area)
Lyre of Building? (another gold statue)
Planar Ally (earth elemental), how much easier can he collect gold and gems?
From Lost Cities of Golarion:
But despite the opulence of its
architecture, the display is mostly ornamental and consists
of leafing and facade work that uses a relatively small
amount of actual wealth. Thassilonians were driven by
greed, not decadence, and stored the bulk of their riches in
well-guarded, heavily warded and trapped treasure vaults
secreted away in the darkest depths of the Hypogeum.
From the RotRL and this entry - I'm thinking that the amount of "A day's work scavenging precious metals and gems from Xin-Shalast's architecture yields 10d6 gp worth of commodities" is actually *using* methods like this.
The idea (I believe) is to get the players to see a dazzling city of riches where every doorknob is a diamond - only to find out it's just costume jewelry - yes you can make money scrounging - but it's not worth the time or effort.