Money without using energy/IAP
Go to an elimination race but instead of destroying the other cars, just drift. Let the others go first and wait 5-15 seconds then go around the track and drift for as long as possible. There is no time limit as long as the other cars are intact. If you can drift well, a few minutes can net araund 1.5 K.
Various Game Hints
You should be steering out of a corner almost as soon as you get into it if you want to clock the best times. Don't be afraid to nudge your opponents off the track. Some even have cliffs you can push them over. With a car with fully upgraded off-road handling, you can take a much more experimental line through the corners.