YabVu - nrps should plan their lives according to how much they KNOW they are supposed to pay in cm (which is actually shameful LOW) just like RP's have to. The child is BOTH their responsibility.
The example you cited of varying mortgage/rent amounts is to a large degree a choice. As are many other expenses.
Maintenance should as pp say be a priority payment.
You also cite benefits being means tested - even this govt accepts there's a minimum amount people need to live on (though I'd love to see THEM try to live on it!) and so the means testing calculations are supposed to ensure people's income doesn't slip below that level.
I'm guessing you're either an nrp or the spouse/partner of one? Or a gf.
It's beyond despicable whatever your position, to try and argue LESS maintenance let alone no maintenance should be paid.
Currently there are far too many loopholes, the agencies set up supposedly to pursue non-payers have been pathetically ineffective.
It should imo (and many others) be a criminal offence not to pay maintenance regularly and in full.
It should also be better calculated to reflect the costs of raising a child rather than just a percentage of nrps salary (which many nrps already deliberately manipulate to appear lower than their actual income especially the self employed).
The nrps outgoings are largely irrelevant. Personally I'd prefer if the calculations were based on minimum income necessary to live on and then the remainder assessed for how much of that can be considered as available for maintenance calculations.