-To easily earn money in Underground mode, upgrade a VW Golf's performance as much as you can. Trade it for a Nissan Skyline, then downgrade it to stock. Trade it in for a VW Golf to earn around $6,000 each time. Repeat as desired.
-Start with the Nissan Skyline, then strip you car down to stock (performance) and trade it for the VW Golf GTI. From there, put all of the performance upgrades back onto your car. Trade your car for the Mazda RX7. You should make about $6,000 to $10,000, depending on what level performance upgrades you have. With that money, you can make an awesome looking car.
-Note: You must have at least $4,000 to do this trick. In Underground mode, purchase the Skyline. Keep the engine stock, the drivetrain stock, the fuel system, stock the turbo, then trade it for the VW Golf. Put everything that you took off on it, then trade it back for the Skyline. You will have made a $15,000 profit and end up with the Skyline.