This tends to be most straightforward issue in Family Law. In most cases, once can simply consult the Federal Child Support Guidelines in order to determine the amount payable.
Complexities may arise when dealing with high income earners, custody/parenting regimes that are "shared" care, when one parent controls a corporation, when one parent is self-employed.
Enforcing support orders (i.e., attempting to actually receive the payment when the payor will not cooperate) is also an issue that may require court intervention.