That's fairly simple. The 1/3 @ $30
Assume 99 carparks (arbitrary)
99 x 8 = $792
33 x 30 = $990
It also encourages people to take public transport. Which I assume is part of it.
But then of course there are other factors, like if the price of parking is discouraging people from going to the stadium, that is probably a bad thing yeah? Or do the stadium not care as long as they maximise their revenue?