Amount of child support is due the custodial parent when the child is with the noncustodial parent?

I heard this topic discussed on the radio. Usually there is no drop in the amount of support. The concept behind this is: the custodial parent still has to pay the same amount of mortage, still has the minor(s) on their insurance, still has the same vehicle, and has to pretty much maintain as if the minor(s) are still there.