City housing is unaffordable. It’s cheaper to buy a house in the suburbs, where you can get more for your money. Right?

Not so much, according to a new study from the Brookings Institute, “The Affordability Index,” which challenges the conventional wisdom by arguing that the best way to assess affordability is with reference to the costs of both the home and the transportation necessitated by the home’s location.

In an analysis of the Minneapolis-St. Paul metropolitan region, it turns out that the suburbs aren’t nearly as affordable as they first appear; nor are city neighborhoods nearly as pricey. That’s largely because suburban residents must spend more on cars ownership and use—they have, on average, 2.1 cars per household—while in-city residents can rely on cheaper forms of transport—they own only 1.2 cars per household. Even when in-city transit is factored in, a city resident spends less than half as much on transportation as a resident of far-flung suburb. That’s real money—roughly $500 per month—that can make a big difference when it comes to affording a house.

But in deciding where to buy (or rent, for that matter), few of us assess the transportation-related costs, a factor which surely contributes to buyers choosing far flung developments. If planners can devise ways to apprise buyers (and renters) of the true costs of their housing choices, it would likely encourage residential density and mixed-use zoning. Because living near good transit service and within a short walk of services isn’t just eco-groovy—it’s smart financial planning.