mmm South Park.. OK.. Forgot about that..
but still thats not gonna get ya to $45 Million. Some of my associates write for some major TV productions on TV each week.. They have money but not $45 million type money.
You've got to figure, though, that South Park has been a big deal for 10+ years and the theme song is there on every episode. It's not just a background score or anything like that. You've also got strong DVD sales, which you don't really get with other shows that have run as long (like Law & Order or something like that). He probably gets another bump from DVD sales of Robot Chicken, too.
It's probably like a smaller version of Danny Elfman's Simpsons income.

