Most states have child support guidelines that provide a starting point. The amount of time the child spends with each parent is factored in, too. If you made the exact same incomes, and the child lived half the time with you and the other half with the ex, then neither one of you would probably be paying support to the other. Outside of that basic number, which is supposed to cover food, clothing, and shelter, you might have additional expenses like private school tuition or music lessons, etc., that again would typically be divided proportionately to your incomes. Or you could agree that one of you pays for school and the other for lessons. You want to be sure out-of-pocket medical expenses are considered, and probably want something in there about college, too, even if it's a long way off. I'd suggest consulting a lawyer, if you don't have one, about how child support is typically calculated and for a list of provisions you will want to be sure are covered in the agreement.