A little googling:
TANF (the last vestige of single-mother welfare) is ~$30bn/yr for the whole US, state & federal contribution.
SSDI (a mix of people with severe medical problems and the work-shy) is $150bn/yr.
OASI (I'm old - gimmie money) is $813bn/yr
Medicare (I'm old - gimmie healthcare) is $709bn/yr
Single mother welfare is a rounding error in the budget compared to other subsidies and welfare, which is mostly diverted towards older and wealthier people.
I'm not just talking about cash welfare -- try holding down a job while raising a kid by your own and see how much our society will bend over backwards to accommodate any lifestyle choice other than single child-rearing.
I don't have a dog in this fight - I'm a middle-aged, childless single guy. But I have eyes to see and it doesn't take much research to figure out who's getting the short end of the stick in our social order.