Let's be specific about what $30,000 means for a married couple with two children.
The Social Security tax rate is 6.2% and Medicare is 1.45%.
$30,000 gross less SS/Medicare taxes of 7.65% (- $2295) leaves you with $27,705.
Income taxes: let's assume you live in a state that doesn't have an income tax. You also have a very simple tax situation, i.e.: no mortgage deduction or anything else. You take the standard deduction of $11,900 plus the deduction for dependents $3,800 x 4 or $15,200. That leaves you with a taxable income of $2900. The tax on that amount is $289. HOWEVER, you also qualify for the EITC - Earned Income Tax Credit of $3600. That means you're getting a refund of $3311.
So during the year, you're living on $2308.75 a month and once you file your taxes, you get a little influx of money.
Let's look at a monthly budget:
Property taxes (if you rent you don't have to worry about this, however if you do rent then you have to pay rent which will be higher. It is better to have a house that is paid off): $100
Homeowner's insurance (let's assume you have a really great deal of $780 annually): $65
Life insurance for your spouse since the worker is covered through the employer: $10
Auto insurance on one car: $75
Auto maintenance: $20 - you use the tax refund for big stuff like tires and brakes
Alternate fuel (propane, natural gas for heating): $25 or you decide to install a wood burning stove.
Telephone (land line plus one pay-as-you-go cellphone): $45
Water / sewer / garbage: $68.75
Groceries - food: $730
Groceries - non-food: $70
This leaves $1600 a month for everything else: clothing, gifts / Christmas / birthdays, entertainment, internet, school supplies, extra-curricular activities for the kids, charity / tithe, home maintenance, medical care, dental care, retirement, savings and every other little thing that comes up. Note, I didn't calculate health care premiums which very soon could easily be $1600 a month for a family of 4.
Yeah, it is do-able but it is tight - very tight. There is no denying how tight it is. As soon as you have an emergency, a car break down, a major - or even a minor - medical problem (having a kid with food allergies could suddenly throw the whole budget out of whack), or if you want to do something like travel or even eat out, you are putting everything else at risk.