SCOTT JAGOW: In a lot of American cities, people have moved a pretty long way from work so they can afford to buy a home or pay the rent. But a study out today suggests we better not forget how much it costs to get to and from that job. Marketplace's Scott Tong reports.

SCOTT TONG: So, what's it gonna be, cheap housing or cheap transportation?

Today's study found that for moderate-income families making $50,000 or less, it's almost always one or the other.

In San Francisco, housing eats up a third of the typical salary. Transportation just a quarter. It's the other way around in Pittsburgh.

Study author Barbara Lipman of the Center for Housing Policy says cheaper cities tend to require longer commutes.

BARBARA LIPMAN: When you get to a more than 12-mile one-way commute, you start looking at transportation costs that exceed your savings in housing.

She looked across 28 cities and found that housing plus transportation costs almost always added up to about 60 percent of each paycheck.

And here's a takeaway point for folks of all income levels: Both items grow faster than wages and salaries do.

In Washington, I'm Scott Tong for Marketplace.

Follow Scott Tong at @tongscott