A fairly cut and dry example of a RP I know...
A couple with a toddler split up. The child was planned. The 'fault' in the breakup lies with the NRP. The RP was a SAHP but goes back to work after the break up in order to support them self. The RP stays single. The NRP turns their back on the child and pays the CMS minimum while getting on with life (social life, new partners, able to work without considering childcare, etc).
RP's rent is 600 PCM. 1/4 of that should be covered by NRP as that's 50% of the cost of keeping a roof over the child's head while not contributing to the RP's rent. NRP should therefore pay £150 PCM to house their child alone.
Child is in full time childcare as both parents work. This is costing £160 a week after the government's contribution of 10hrs. £80 a week of that cost should be covered by NRP. That's £320 a month.
That's £470 alone that the NRP should be paying. They're actually paying £250. RP is however getting some tax credits to top up the money they don't get from NRP so that they can afford to go to work.
That still leaves the child's share of food, water, heating, electric, council tax, as well as toys, nappies, entertainment costs, birthdays and Christmas, transport, furniture, shoes, etc. All of these are covered exclusively by RP. When the child gets older this will also include gifts for friends' parties, clubs and activities, school meals...
And as if that isn't unfair enough, should RP want any form of social life they also have to pay through the nose for babysitting. And their career suffers because when the child is sick, they can't go to work.
I don't see how the RP could be called a money grabber, bitter, etc, considering the circumstances. But that's what they get called if they have the nerve to complain.