Ya I beat the game already so now i'm trying to unlock all reward cards...

You do know that there's an interesting machine called Action Replay/GamShark..you can use that one to make money otherwise just keep re-racing.

Hey there, I also completed the game and then only thought about the reward cards such as career mogul (get $1.000.000) but the best way to do it is: start a new career and choose the tuner side (mazda rx8) then, don't spend any money at all,(although my friend got nos on his car) race only 2 races in each area so that youve got the area but theres still 1 race left in each, then beat Kenji and race with his car in angies area and remember, only 2 in each area and don't spend any money, once youve beaten angie, put samson as your active crew member because his race bonus is 10% extra cash(make sure neville is ALWAYS in your crew) now you can race all the left over races and Kenjis car (rx7) will be good enough how it is to beat all races up to darius.
Now, if you followed my instructions you should have more than 1.000.000 but, don't go spendin it just yet, sell all your cars (if any) except your rx7 and rx8 and go buy all the rx8's if normal edition or nissan 240sx's if collectors edition till you have a full garage, you should get a card for that. Then sell all except one and buy the cheapest muscle car till you have full garage. But youll have alot of muscle cars and 1 tuner so sell the tuner and buy the last muscle and repeat with exotics(you should get reward cards for fillin your garage with only tuners, muscle and exotics) then sell all except one. You should have Nikki in your crew as she gives you 10% discount on all cars. At the end you should still have between 900.000-1.000.000 in your bank acount. Now go buy your dream car (i excpect you to get the nissan skyline, lol) and tune/upgrade it as much and best as you can and go kick Darius's a**!

Thanks to PaineIsCool who told me this.

Hope my long speech helped, bye now!

I found it much easier to do if you start of as muscle. You only have to beat 21st street angels to get Samson, who gives the 10% bonus to races. Less races you have to waste w/o the bonus.