After defeating the spider boss in Everlight Temple you will get back out using the elevator.
Once you have the Netherworld Gate to this zone unlocked, return and open the
secret room at the top of the stairs (using the Overlord switch). The
treasure chests inside this secret room will give you an average of 1000 or more gold each visit. You
will need to warp to another gate to reset the map. Try Nordburg Town gate 3,
as there are four or five treasure chests right next to the Tower Gate.
Rapidly cycling these two maps should inflate your treasury with plenty of
gold.