You are not taking into account the expense of kids in daycare or any kind of emergency or home maintenance. It's not every month but houses need regular maintenance and it's usually not on your personal savings schedule. Our water heater just needed replacement, that's $1500 right there. Fridge crapped out, that's $2000. That's not even getting into the really expensive stuff like HVAC, roof replacement, etc. That combined with the costs of kids is going to eat up that extra $1300-1800 real quick.