You can't gain any extra prize money after completeing gold on teh special events. You can redo A-Spec races and gain the prize money, but no tthe prize cars.
Would be nice if we could clear the achievments and redo them to gain the cars like in GT2.
If you really wanted I would play GT5 on your other PSN account (I know most of you have one for the GT5 demo) and just start a new game. Then when you win anything, gift it to yourself.
And alot of the cars that you win you can actually buy. I won a '97 Premium Civic which I had already performed a Racing Modification on one I had previously LOL.