Easy gold on "Experimental Facility"
On Amateur difficulty (for easier kills), continue until you get to the part where you are in the hallway with all of the Lickers on patrol. Go through it and kill all of them while picking up their Lion Hearts as you go. This is easier when you use a fully upgraded Magnum or rifle because it usually results in one to two shots to kill. When you get to the other end of the corridor, do not open the door. Instead, pause the game and go to quit, saving, and overwriting when prompted. When you reach the main menu, go to "Item Management" and sell the Lion Hearts. This usually earns you about 12,500 gold each run. Go to "Continue" and restart at the beginning.