I agree with Margaret that it is normally based on the DSS Child Support Calculator. If the custodial parent earns significantly more than the non-custodial parent, and the non-custodial parent has 110 or more nights of overnight per year (known as "shared parenting"), then it is possible for a non-custodial parent to receive support. The rationale is that the child is spending enough time with the non-custodial parent that the court does not want the child to have such a tremendous decline in lifestyle on those evenings at the non-custodial parent's house.
If someone attempts to apply the Child Support Calculator on his or her own, I encourage that person to read the Child Support Guidelines, also found on the DSS website, before doing so. Using the calculator is more complicated than it seems, which is apparent after reading the guidelines. A family court attorney could help ensure that the calculation is being made correctly. The fact that the non-custodial parent's income is not documented makes it even more difficult.
In addition, to change a previous order in the family court, the parent seeking the modification must normally show a material change in circumstances from the time the order was entered. There is an enormous body of case law in South Carolina explaining how such a showing must be made. The single mother's family court lawyer would need to know the parties' situation at the time the order was entered and what is different now. Changes can include a decreased income for the spouse paying support or a decrease in overnights with the child for the party receiving support, although there could be many other factors supporting the change.
I am concerned that the single mother on disability does not have a high enough income to warrant paying support. If able to modify the order, not only could she eliminate the payment she is making but could possibly begin receiving payments.
The foregoing is for general information purposes and does not establish an attorney-client relationship.
That's certainly atypical, especially if the non-custodial parent has no overnight visitation. To see if that number should still apply, you can go to the SC DSS website and run the numbers under their child support calculator. If it doesn't match up, you may want to consider filing an action to modify child support, which you can do pro se (without a lawyer). Just check with the Clerk of Court in your county for the appropriate forms.