Having them pay to fix a car wouldn't work though, you know how people are with some games. People will cheat, get a ton of credits, and go have fun. They will exploit the system and thus making it not work. The best way to deal with it is to have PD figure out a better time penalty system.
regardless of how many credits a rammer has, if they get ex amount of time offline, like a ban for their pathetic driving. When their ban expires, then they can play online again, provided they pay for damaged caused. If they don't want to pay, then they wont be able to play online.
Casual gamer will always be the most import for a game company, that's where they make there money. GT is one of the best selling series on the Playstation as far as I know, it takes more then a few thousand hardcore people to get those sort of sales.