To collect unlimited amounts of gold, after defeating the Spider Boss in Everlight Temple use the elevator to get out. Unlock the Netherworld Gate to this zone, then return and open the secret room upstairs using the Overlord switch. There are treasure chests inside this room that will give you around 1000 gold every time you visit. Warp to another gate to reset the map, then return to collect it again. Repeat as desired.
Enslave both Nordberg and Everlight for large chests of gold that spawn when the area is reset. Then, teleport between the two locations to collect from both of them and reset the other area for your return. Each set of chests has at least 1,000 gold. Note: You must completely dominate a town to get 1,000 gold from the chest spawning sites. All 100 citizens must be enslaved--for each 10 citizens not enslaved, 100 gold will be subtracted.
After the assault on Empire City, return there to find no troops, several boxes with the highest grade armor and weapons, and over 2,000 gold per visit.