1. Go to Arcade and Quick Race
2. Go to the Retro tracks (unlocked after beating the single player mode) and pick Game Mode 1.
3. Set Number of Laps to 10 and Difficulty Hard.
4. Pick any car you want.

Every Perfect Lap you do gets between 1500-3500 Credits depending on the car class you chose, so you should get about 16,000 to 30,000 depending on how fast you get into first.
And the best part: It takes less than 2 minutes!