What glitch did you use? I read about an offline one like Astroman mentioned, but it seemed like more trouble than actually racing...
It was more trouble than racing, so it's probably the same. It involved where the game saved and where it didn't.
In FM2 if you were upgrading a car and you didn't like the parts you'd bought you just hit the Xbox Guide button and backed out and reloaded and nothing had changed. Well, in FM3 I bought a bunch of parts for a car and didn't like how it was turning out, then I noticed there was a "reset to default" option so I did that thinking that it would unbuy anything I'd bought. Well, I was wrong, it just uninstalled the parts. BUT it also saved the current car setup when you did that. Well, I figured I'd go the FM2 route and hit Xbox Guide and reload so I did so. But when I booted the game up and went to upgrade the car again I noticed that I still had all the parts I'd bought previously AND all the credits.
Yeah, it was a time consuming process but it took maybe 5 mins to buy
all the parts for say, the Ferrari 355 Challenge (most valueable parts I could find), quit out, reload, and sell them. At ~$450,000K/pop it was much faster than racing.
Definitely not worth the time spent doing it but for those credit crazed it was effective. I'm glad it's gone.