This is easy, assuming an average American family size:
- $8,333 gross ($100k)
- call taxes 25% (this is aggressive) leaves $6,667
- housing expense of $2k leaves $4,667
- minus 2 auto expenses (used car average payment is $465) brings us to ~$3,400
- childcare (which: is not free even when your kids are in school) of $1,500 yields $1,900
- frugally feeds the family on $1,000 per month
- $900 remaining for everything else. This includes all the one-off incidentals (car tires, physician co-pays, etc.).
The astute reader will have noticed that I was generous in my tax, housing and childcare estimates. The more astute reader will note that I have omitted student loans, any healthcare or dental expense, vacations, etc.
This is obviously not an estimate for a HCOL area, this is what the math looks like in much of the country.
Edit: _The Two-Income Trap_ is a good read on this. IIRC it points out that most of the big costs are fixed, where financial gurus often tell people to cut the variable expenses (cut down on avocado toast, or make coffee at home). When obviously tweaking inside the $900 left over won't fix this budget no matter what.
Also worth noting that even if they could halve their housing expense, they still would only be in a position to put away $12k annually. Which, given what we know about unexpected expenses, would probably put them right at actual break-even and prevent them from taking on consumer debt.