There is a second money cheat that still works (?) for those who have not finished the island cups on Oahu. Especially in Area 1 I think. Basically all you have to do is win all races in that Cup and at the end not press "next" but "retry". You get 2.5 million then, and when you re-race on of the events ( choose the time trial, just wait at the start until the time runs out ) you may not win again, only get in second place or worse.
You can only do this once. That should give you about 2.5 million in under 3 min. Remember, one sitting only. So you better do this for 30 or 60 min then you shouldn't run out of money.
I didn't try it myself so far, for more info go to the forums of
http://www.xbox360achievements.org/
Oh and if you have finished all cups already there is no way to use the trick above.