I am not married with the Mother, and she screwed me financially before she left... I can't live with half of my pay garnished!
Family Law Attorney
It depends on your income, and her income. (There's a complicated mathematical formula; lawyers and judges mostly use software to do the calculation, since it also requires figuring out the income taxes).
For example, with a 50/50 time-share, if you gross $10,000 per month, and she grosses $1,000 per month, you'd owe about $950 base child support. On the same income numbers, you'd still be required to pay Mom some child support, even if the child was in her custody only 20% of the time.
If you and she have a 50/50 time-share, AND you both make exactly the same amount of money, AND neither of you has any other children, etc., THEN there'd probably be no child support due.
The amount of child support is determined by how much you make, how much the mother makes, how much time each spends with the child, and several other factors. These issues will be taken into consideration when the courts determine how much and who pays child support.