Obviously the description of this trophy is pretty self-explanatory. You can earn money by pilfering treasure boxes you find on your adventures or by completing the "jobs" on the bulletin boards at safe-houses. You probably won't be able to spend all $5000 in one sitting, just spend what you can and go back. You can spend money at the weapon lockers in the outposts or at the few weapon shops scattered on the island.
*Note- you can try to win money by playing poker, but you will lose more than you win, trust me.*