It's between Like The Wind and American races at Indy. I only had a chance to do each one once before I left for work and didn't write anything down, but it's very close between those two.
I **think** American took a little more than a minute longer (using my Spec II Test Car on American vs x2010 at LTW) for about 25k more credits (99.xk vs 75k). When you consider the short time that it takes to get a new race going, they seem really close. I'd be interested to see calculated numbers if anyone has them though.