Williams and her family live in north Philadelphia. She spends her check only on the essentials: rent, gas and electric, bus passes, a phone. She doesn't have cable or internet.

"If you own a home, plus childcare, plus commuting costs you can be well above poverty and still not be able to make ends meet," said Professor Scott Allard, an expert in poverty and the social safety net at University of Chicago. "You're not doing anything wrong. You're playing by the rules but you're not making it."



Time is money



The expression "time is money" is especially true for the poor.

Those who earn little have to work long hours to make enough, and often spend more time than the well-off managing the basics of their lives. Not having paid sick days means losing wages when a loved one has a medical emergency, or child care falls through. Not having a washing machine means extra hours at the laundromat. And just getting to and from work can take hours.

Every Monday through Friday, and sometimes on the weekends, Williams leaves home at 7:20 a.m. and takes three public buses to school. She arrives at 9 a.m., and finishes at 2 p.m., sometimes lingering to finish homework. Then she leaves for work, taking three more buses to get to her client's home in West Philadelphia.



(Read more: What is'wealthy'? $5 million and plenty of cash)

The trip back to her house means two more buses, and arriving home around 10 p.m.



The 28 hours a week, spent waiting for, or riding, the bus, have become another part-time job. Because the agency that employs her requires she pick up her paycheck in person, she takes another two-hour, round-trip, bus ride every two weeks.



"I pretty much spend most of my time on the bus," Williams said.



If Williams had a car, she could cut her travel time significantly. Low-income individuals and families in desperate need of a car often turn to so-called "buy-here-pay-here" car dealers, which typically offer high-mileage, used cars and in-house financing to those without good credit. Loans from these outlets carry an average interest rate of 24 percent, according to the Center for Responsible Lending, and the industry is little-regulated.



Health care



Studies show low-income people pay a larger share of their incomes for health care. Low-income workers are less likely to receive employer health insurance, or even sick pay, making a day off more costly. Medical care can quickly become medical debt, which can hound people for years, if not a lifetime.

While public programs like Medicaid and CHIP provide health care for low-income individuals and children, getting enrolled and staying on those programs can be tough. Moreover, many working poor earn too much to qualify for Medicaid, but not enough to purchase coverage.



(Read more: Top 1% Control 39% of World's Wealth)

That's what happened to Williams two years ago after she lost her job as a bus driver. On unemployment, she applied for coverage through Philadelphia's Health Care Partners, a Medicaid managed care organization. She was told she received too much unemployment income to qualify. When her unemployment benefits ran out, she applied again. A paperwork snafu delayed their enrollment for nearly a year. The delay had real health effects for the couple, both of whom have diabetes, a disease which disproportionately affects low-income people.



"We couldn't afford to go to the doctor," she said. "We didn't have medicine. We were under control when we had medical insurance."



Banking also is more expensive for poor people.