While playing Racing Life, an easy way to get new cars without buying them is to go back into races where you can choose cars. Then, all you have to do is pick a car you don't have and win with it.
After you defeat Nelly in a drag race in event challenge mode, you will get his Mazda RX-7. Keep repeating that race, and when you win you will get over 100,000+ RP every time. Repeat this as many times as desired.