Child support guidelines are based on a percentage of what the parents make instead of what actual costs are

Current child support guidelines are based on the income of the 2 parents put together. Shouldn't child support be based on the cost to raise a child and then divided in half to be equally shared between both parents. As current guidelines on child support if one parent has custody and the other has more income then the other, the non custodial parent has to pay 80% of child support to the custodial parent. Why should a non custodial parent have to pay more of the child's expenses? Lets make this fair to all and have child support based on the cost to raise a child per year instead of what type of income the parents make!