Good day, Welcome
The fastest way after finishing career that I found to make serious coinage is to repeat the Rally mode championship that pays 500,000coins, it is called MASTER OF RACES. On average to finish the 15 Races it will take anywhere between 30-35 minutes (give or take for replays) why rally mode? In my opinion these cars handle the best as you don't have to cater for understeer or drift.
Personally it took me 5hours after finishing career mode to do this, to be honest all car types where the 15 race championships are concerned will take the same amount of time so it is your own personal preference. If like me, you actually finished 5/6 career types (not the impossible to handle sports cars, right?) and won them all then you will have earned 4,250,000coins leaving 5,750,000 which if you then grind out the remainder should take you 5-6 hours, 500k per half hour = 1m every hour x 6 =6million coins.
Good luck and have fun playing this little gem of a game.
P.S let me know in the comments if you can master the sports car mode, they're so darn difficult to control
I say this because I was thinking that maybe_just maybe there is a bonus pay out for completing/winning all championships as opposed to the 5/6 that some people will have because of the sports cars handling, its a long shot but I might give it a go and post back. Anyway happy grinding.