After completing career mode make Samson your wingman and put Neville in your crew. Now you will receive $750 rather than $500 when you repeat or start a new race in which a wingman is used.
The easiest way to break 1 million in Career mode is to start a Muscle career and race two races in order to unlock the car lot. Afterward, go to your challenge series and unlock the Nissan 240 (last set of checkpoint challenges). Purchase the car in your career. It is possible to complete the entire game with this car. It is important to start as a muscle career so that you unlock Sampson first. Race the entire game with Sampson set as active and Neville in your crew. You will break 1 million with about ten races left over.
Go to the Billings District, Downtown (tuner area) and to the Mason Fountain race (speed trap race). Do not join it. Instead drive south of the race and find an entrance to your right. Take it and you will go off a ramp. Once you go thought the entire shortcut, turn around and look at your map to notice an orange arrow. Race the car to any location. When you win, you'll get $1,000. Drive back to the jump and do the same thing again. The orange arrow should appear again. Race the car again for another $1,000. Repeat this as many times as desired. Note: This may take some time.