Hint: Easy money:
Choose a car you wish to gamble for but do not press "Gamble" yet. Instead, remove your memory card, then press "Gamble". You will be asked if you are sure. Select "OK", then the message "Autosave has been disabled" will appear. Select "Continue without autosave". If you lose the race, reset your Gamecube, insert the memory card, and you will still have the money. If you win, put the memory card back in after the race, then go to options and autosave. Select "Overwrite file", and you will be able to save the car again.