I ran it several times last night but only got credits for the first win. After that I got nothing.
If you finish 6th the first race, 5th the next and so on I think you can get the most money this way.
Its the same as special events or licenses, you get rewarded for the position you finish in plus all the rewards for any positions below your final one so if you win it you get all the cash available and wont get anything for re-running it. if you come second you'll get second and everything below, then if you re run and win you'll get get the 1st to 2nd prize money but nothing else.