Yea I think the code you are refering to can only be done by gameshark. GT3 really doesnt have that many "cheats."
IMO using that code to get you that much money in GT3, would seriously be a total waste of money to buy the gameshark, although it does work for a lot of different games. Just play GT3 through, its the best racing game known to date IMO, dont screw it up by using crappy gameshark codes.