Where do you live? This is HUGELY dependent on geography. $18k a month in NYC, Seattle, SF, etc., is pretty painful. Add two kids, private school (unless you live in a great district), college educations, saving for retirement, summer camp, braces, health care for 4, housing for 4 in a good neighborhood, helping out your retired parents who might not have saved enough, etc., it stacks up pretty fast.
$18k/yr in NYC or SFBA with a mortgage and a family is untenable.
$18k/yr in most major US cities probably implies:
* Subsistence diet
* Dependence on small selection of rental properties
* Extremely poor access to health care
* Dependence on informal "gratis" child care
* Near perfect job attendance
I call these out not to make the boring "gnash our teeth about the lives of the poor" case, but rather the suggest that your ability to care for a family on 18k/yr in a US metro is counterfeit: you can do it until you:
* Are ever hospitalized
* Lose your free child care
* &c ...
... at which point bang you're bankrupt. I think a lot of people who think they are getting by with low incomes are actually playing a kind of sick inverted Martingale betting strategy against life.
Well my point is not that everyone should be able to live off $18k a year. My point is that You don't need $500k a year to live decently. I have a new car, a good apartment and amenities such as cable TV and xbox live. Certainly geography plays a large role in individual costs. But again, I'm not trying to say everyone should live on $18k a year. I'm saying I (a single fellow btw) do, and it's a satisfactory lifestyle complete with all the necessities and a few luxuries.