I would think that the F1 race is by and large the most profitable event on their calendar. Although the NASCAR weekend has a slightly larger attendance, more money is spent overall on F1 weekend I'd have to imagine.
NASCAR doesn't even come close to F1 at COTA.
According to the Austin Statesman, NASCAR's Sunday attendance was 50-55,000.
F1 easily has well over 100,000 for Sunday, especially when you considering the weekend attendance was supposedly 440,000. The math alone means Sunday was well into the six figures.
Interestingly, the article says SMI is renting COTA for the NASCAR weekend. So COTA isn't even promoting the event. It's SMI.
Thus, COTA only has two events they pay to put on -- MotoGP and F1.
As for F1, let's do a quick back of the envelope figuring. Weekend GA tickets cost $380. Grandstand tickets cost way more than that, anywhere from $600 to easily $1,500. That's not even counting the VIP areas.
So, let's say for quick grins and giggles, the average ticket price was $500. Probably way lower than reality, even when you consider not all tickets sold were weekend tickets. Multiply that by 440,000, and you get $220 million just in ticket revenue. And that's before you count parking, concession sales, and sponsorship.
Sure, there are a lot of costs putting an F1 event on. But still...