That probably depends on which state you are in. I am in Iowa and joint custody means you figure the support as if the W was the primary custodian, and then figure it as if the H was the primary custodian. Then you net the two numbers and the higher one pays the support.
For me (and this all depends on how you figure the net income - who gets tax deductions, credits, etc)... I figured I'd pay at least $750/month to my W if she had primary and she'd pay me somethign like $125. So I will have to pay at least $625. How the hell is that fair?