I've been playing GT5 for some time now and reached levels A-Spec 25 B-spec 31.
I've recently found out that by participating in the extreme B-spec-extreme Series-Dreamcar Championship and enter the championship series for all 5 races with the Red Bull X2011 Prototype you can make around 1,732,000 cr every 1h and 30 min or so depending on the cars settings and weather in each race.
looking at the prize money for placing 1st in one of the races it has 86,600 cr as your prize, but after you finish the race you will actually win around 173,200 cr per race instead due to the bonus you get for being logged in to your PlayStation network id.
(Also on a sidenote it's also a good way to grind exp for your drivers and B-spec levels.)
After placing 1st in all the races you'll average around 866,000 cr just on the races alone. then for completing the championship and get the gold trophy you get around 866,000 cr on top of that giving you a grand total of 1,732,000 cp.
To tell you the truth about it. It didn't take to long to complete the championship because the X2011 Prototype screams through the courses. Here's what I got on the time it took me to finish.
1st race completion in around 14 minutes.
2nd race completion in around 18 minutes.
3rd race completion in around 13 minutes.
4th race completion in around 14 minutes.
5th race completion in around 18 minutes.
That adds up to 1h and 17 min. Add load times between races and if it's raining on the racetracks you'll end up with 1,732,000 cr every 1h, 25min to 1h, 35min.
(PS. I was logged into my PSN Id while playing and had exp and prize money bonuses in play.)
So give it a try and tell me what you guys think.