And if you've got all the one-time prizes, the best money grind right now seems to be the last Supercars race. Jump in a detuned Le Mans racer (I had the Peugeot 905 handy, plenty of choices though) and for about 5m 30s you can get an easy 490k.
And with the x3 prizes for all the seasonals I'll probably be rerunning a bunch of the older ones too with different / more competitive cars, you just can't beat 200-300k or more per race!