Nope, its really quick to get credits...do the following method:
Go into Options/Misc, and enable the AI uses player setup
Then in single player, make a VERY *horrible* setup (i had mine so they move at 1 mph), and add a few AI bots
Change difficulty to hardest setting, and set laps to 3.
Then start the match.
As soon as the screen opens up, press escape, and go into Pits
Choose a new setup for yourself (a good one)
Every other car will remain the ****ty version...
Whala
No competition, and 48+ credits a pop.