I just run a metric-ass-ton of driver's battles until I've saved up enough to buy the car and start modding it. I think I've got about 1.6mil saved up right now. Earlier in the game though I was selling my car after having built up a savings and using the combined funds to buy the next level car. I could sell my Aero TT right now and make enough to buy a Venom and mod it to everything at least 2 blocks full. I don't have enough RP right now since I JUST bought the tuning package for the Aero. (1945 RP's)
Once I have a car modified enough I start running face to face and betting races and that's where your RP's really start to accumulate quickly.