quickest way is to do the license/challenges...as it does give you a fairbit amount of money (gold, silver, bronze, assuming u get gold) in the shortest amount of time. I believe someone got all gold and was left with roughly 11,xxx,xxx - 12,xxx,xxx.
Another way is to go againts someone in adhoc mode, make sure to keep playing and not quit the lobby, as during the race, thee "JACKPOT" will increase its multiplier (it went up to x8 on me). Nurburgring will give about 100,000...calculate jackpot and u get up to 8xx,xxx, on top theres a "lucky" player