You will be able to obtain this trophy once you reach the Dwarf Tunnel part of the game. It is a missable trophy, so be sure to read on. This is shortly after the bridge is blown up by two mini-bosses, and you return to the pier-town to ask questions on an alternate way.
Once you're in the Dwarf Tunnel, you'll first be in the NPC-camp with shops, etc. Take the opportunity here to sell some of your goods as you will need 5000 gold. Around this point in the game, I had 4300. I sold a bunch of Magic Items, and regular items to get 4900, then I begun to run back and forth between areas to kill enemies so I could get more gold.
Once you have 5000 gold, progress past this NPC-camp by talking to people and going beyond the rock-wall that's broken down for you. You'll come across Wyatt as part of your main objective, who will ask for 5000 gold for his nitromyte. You have two options, agreeing, and disagreeing. Not having enough gold/disagreeing will progress in the story, making this trophy missable. So make sure you have 5000, then agree.
If you don't want to sell your items, then make a save when you're in town, sell everything you have, go forward and talk to Wyatt with your amassed sum of gold to get the trophy, then revert back to your previous save.