You must possess the 50,000 at one time, so as you play through the game be thrifty with your upgrades. Best done at the end of the game when you have most areas unlocked. If you're a domination Overlord Nordberg Gate 2 has a lot of Chests right beside the gate; so does Everlight Town Gate 1. Simply cycle between both of these until the chests become smaller. They can only be collected so many times before this happens. The number of them depends on the population of each town.
The next fastest method is to gate to Everlight Temple. There is a switch up the stairs to the left of the gate as you exit that unlocks a secret area. There are also 5 chests close to the walls on the way up to that room. Each run should net you about 1000-1400 gold.