In the US, every jurisdiction uses a formula to determine child support. It takes into account how much each parent earns, certain expenses (like medical insurance premiums), the number and age of the children, and any special needs of the children.
Numbers are plugged into a formula, and out spits the amount each person is obligated to pay (or receive).
Most jurisdictions have online child support payment calculators. Try doing a Google search for your state. You'll probably find the form and be able to come up with a good estimate of each parents portion.