500cr in the Japanese version. Since the Credits in the Japanese version are scaled like yen, then that'd be like 5 bucks, or 5 cr in the US version assuming the US version gauges the credits to match US dollars.
Oh, and that's 500cr eveytime you want to drive on any of the tracks in sim mode that is not part of an official race. Basically, you got to one of the course hubs (City course, Original course, Real circuit, dirt and snow) on the main menu. 500cr sounds like a lot, but really, it's chump change. hell, chaning your oil or getting a wash costs 5,000cr. A new set of wheels costs 50,000cr. (just for comparison, the very first race, the Sunday cup in begginer hall awards 60,000cr for first place. After that prize credits goes up 100,000>150,000>200,000>300,000>750,000....