$1 million
Note: A Dreamcast keyboard is required for this code. Use the keyboard in port D. Type "goldfinger" during game play. A sound will confirm this portion of the code. Then, type "givememoneyordie". A sound will confirm correct code entry. Exit the race and you will have $1 million. Repeating the code will set your current amount to $1 million -- you cannot get more than that amount.
Arctic Wasteland: Hidden money
Go through checkpoint 2, but continue straight instead of following the track. Pass between two hangars and continue straight across a small lake. Ahead of you will be an American flag. At the base of this is another hangar. A money box is next to the hangar.
Arizona: Hidden money
Go to checkpoint 6 -- it is easiest to do a 180 at the start line. Go straight out over the tracks over the mountains and find the little town that is out a bit.