"Missouri actually passed a law that divides the child support between the parents so that each contributes, yet there is no oversight of how the funds are spent."
Mass is similar. Both parent's income is taken into consideration -- sort of. The custodial gets a $20,000 per year offset (in other words, the first $20,000 doesn't count plus any daycare expenses come off too). Then they apply a percentage to the total income of the two; the percentage increases with income up to about 33% for most white collar workers. Then they add a percentage depending on the age of the kids. 10% if they are 13 - 18 (23 if student). Then this basic order is adjusted downwards by the percentage of total income that the custodial makes. Then the family health insurance comes into play; if the custodial is paying for it, the CS order goes up by 50% of the premium, if the noncustodial is paying it, the order goes down.
And they use gross income, not net.
Bottom line, with 3 kids, if the non custodial makes say $50,000 and the custodial $20,000, the CS is about $300/wk plus or minus 1/2 the medical insurance. If non custodial earns $100,000 and custodial earns $50,000, the CS will be about $515/wk. This does not reduce the custodial's income tax bill, so if the custodial nets 65%, the $50,000 earner would be left with $17,500 net; the $100k earner would be left with $38,220.
The custodial parent in the lower income situation is probably netting close to $18,470 from work plus $15,600 from CS for total net of $33,100. (Note that new lover/spouse doesn't count for custodial, it might for non custodial). The higher income situation results in an approximate net of $40,000 from work plus $26,780 from CS for a total of $66,780 after tax.
It gets interesting if they earn the same amount; if they both earn $50,000, the non-custodial will pay about $200/wk after adjustments but before health insurance. If they both earn $100k, the non-custodial would pay about $375/wk.
Both families would like have been better off financially if they stayed together.