In Georgia, there's no simple answer to this - it's not a set amount per child, or a percentage of the payor's income. Instead, a base child support amount is calculated by a very complex formula and then altered as needed based on other circumstances. To calculate that base amount:

We look at how much both of the parents make, combined.
We look at how many children there are.
We use a predetermined amount for how much people making that amount of money are expected to spend on that number of children.
We divide that amount between the parties, proportionate to their income.
We look at any other particularly expensive needs the children have, such as private school tuition or serious medical costs.
Other adjustments are made for things like which parent pays for the children's health insurance and any daycare.
If the parents have evenly split custody, the child support amount, if any, will be reduced.

After adjustments are made, the non-custodial parent pays their portion of the expenses to the primary custodian.

The basics of the formula - each parent's income and the number of kids - are straightforward enough. The attorney's skill comes into play when looking through other factors to establish whether the base amount of support should be adjusted up or down, and making arguments for and against adjustments to the judge. An attorney can also help you collect past due child support, have child support automatically come out of the paying parent's paycheck, and help you during a modification of child support after the initial decision.