How much does the non-custodial parent have to pay for child support?

If the noncustodial parent tries to keep the child, the custodial parent can get the noncustodial parent charged with kidnapping and contempt of court both can be jail time for the noncustodial.

It is assumed you mean the parents have joint legalcustody and one parent has physical custody.Generally, the parent with physical custody is awarded the child support since child support is intended to help pay for the child needs, living expenses and all the associated costs of raising the child. The custodial parent has much more in living expenses that are associated with raising the child.See related question link.

The custody order must be modified by the court. If the custodial parent will consent the change will be much easier.

Depends on how much the parent to pay child support is worth/currently makes. Varies largely state-to-state.

A friend of mine had the same issue. He paid some of the child support. When he and the mother went to court, the court looked at how much he earned at the time and the time of the child support claim. The court determained how much he had to pay every month and deducted the ammount he paid the previous months and what ever was left they added on top of his current child support for a period of time. After he paid the amount which was owed to the child he started to pay the regular amount set by the court.