I do.

Im level 31 A spec and 30 b spec.

For easy cash do special events they are all damn easy, expet vettel X Challange wich is actually a bitch, but not impossible (gives you 250k Cr for bronze).
Another easy way is to enter "Like the Wind series" and race at Indianapolis (unlocks at level 20) or if not just do the "Supercar Series" and spam speedway Daytona.
For Indianapolis use a Corvette Z6 with racing modiications done. For Daytona just use a tuned Camaro and it will do the job.
Check my GT5 channel if you wish. Its currently down as PSN is down too but w/e.

https://us.gran-turismo.com/us/gt5/user/Wildlash/

Btw Vettel X channalnge gives you the best car in the game. Click spoiler to see it. (I have it). You can also get it by achieving evel 35 B Spec.