Start grinding this one after you're done with your career. Even then you'll be halfway to 10 million at best.
In the GT1 section select Hockenheim Day & Night. Drive the first two races like you normally would. When you have raced and won the third event select "Play Again" by pressing
. Repeat the race for as long as you can stand the game. By doing this you will not only get the money for first place everytime but also the bonus cash for winning the tournament. Make sure not to back out, otherwise you'll have to start at the first race again.
It is enough to do this on easy difficulty. Everytime you complete that race you'll get:
- 23.000 for winning the race
- 34.500 for winning the tournament
57.500 per race. Not bad for ~5 minutes. If you do it on a higher diff you'll get slightly more of course. Don't be fooled though. It will still take quite a while to get to 10 million.
As for the car...I went with the Chevrolet Corvette C6.R GT1. The Maserati MC12 GT1 is faster but the Vette handles better. Your choice in the end.