This is take from Ign.com:
Thanks to some reader submissions, the best way that we've found to make a lot of money quickly is to enter the Amateur League European Championship. It's a five race with only three laps per race, so it can be completed in less than 20 minutes. You need to finish first in the first four races, earning 7,500 credits per race, then save your game. Enter the fifth and final race and simply quit out of it to end the series. If you finished first in the first four races, you'll win the series, more prize money (30,000 Cr.) and one of four random prize cars. If you get the Gillet Vertigo Race Car, you can sell it for 300,000 credits, thus bringing your 20-minute prize total to 360,000 credits. If you don't get the Vertigo Race Car, just re-load your saved game and exit the race until you win the Vertigo. Doing this, you can net yourself nearly 1,000,000 credits per hour.
The only problem here is that you're going to need a car that's at least 500 HP in order to compete. For those at an earlier stage in the game and looking for an easy way to make some bank, here's another suggestion. Get a car that you've tweaked out to where it's around the 300 HP range and pretty quick. Enter the Super Speedway race in the GT World Championship in the Beginner League. The race will take you a little less than four minutes and you're awarded 10,000 for finishing first. This'll net you an easy 150,000 in about an hour, which should be enough for you to buy something that you'll be able to tweak enough to race on the Amateur League European Championship.
