Washington ? How much does a gallon of gasoline cost?

It seems like an easy question. You might ask whether I mean regular or premium, and where in the country I’m buying. Beyond that, though, the price is displayed in giant numbers on most main roads. It’s such common knowledge that we ask politicians to rattle it off to show that they retain some minimal awareness of the world they claim to represent.

But as the sludge choking the Gulf of Mexico shows, nothing is easy when it comes to oil. Not even the price. In fact, especially not the price.

Most of us would call the BP spill a tragedy. Ask an economist what it is, however, and you’ll hear a different word: “externality.” An externality is a cost that’s not paid by the person, or people, using the good that creates the cost. The BP spill is going to cost fishermen, it’s going to cost the gulf’s ecosystem, and it’s going to cost the region’s tourism industry. But that cost won’t be paid by the people who wanted that oil for their cars. It’ll fall on taxpayers, on Gulf Coast residents who need new jobs, on the poisoned wildlife on the seafloor.

Hidden costs

That means the gasoline you’re buying at the pump is — stick with me here — too cheap. The price you pay is less than the product’s true cost. A lot less, actually. And it’s not just catastrophic spills and dramatic disruptions in the Middle East that add to the price. Gasoline has so many hidden costs that there’s a cottage industry devoted to tallying them up. At least the ones that can be tallied up.

Topping that list is air pollution, which we breathe in whether or not we drive. Then there’s climate change, which is difficult to slap a price tag on because it involves such esoteric calculations as how much your grandchild’s climate is worth. There’s traffic congestion and accidents, which harm drivers and non-drivers alike. There’s the cost of basing our transportation economy on a resource that undergoes wild price swings.

Some of the best work on this subject has been done by Ian Parry, a senior fellow at Resources for the Future. His calculations — plus some data from other sources and studies — suggest that adding all the quantifiable costs into the price of oil would increase the cost of each gallon by about $1.65. According to the Energy Information Administration, the average price of a gallon of gas was $2.72 last week. It should really be as high as $4.37.

Incalculable costs

That, however, is almost certainly an underestimation. There are plenty of costs that we just don’t know how to put a price on. How much of our military policy is dictated by our need for secure oil resources? How much instability is created by our need to treat oil-producing monarchies such as Saudi Arabia with kid gloves? How much is the environment worth in a poor country that would prefer oil investment to air-quality regulations?

Or take the spill in the gulf. What’s the economic value of a whale? Of a pelican? Of plankton? The nation’s been horrified by the photographs of oil-soaked wildlife, but how much is not being horrified actually worth to us? And is not knowing about the problem enough to solve it? One reason we’re drilling wells far offshore and in countries with poor safety and environmental records is that we don’t want oil companies mucking about in the shallow waters near our homes, or on public lands such as the Alaska National Wildlife Reserve. But as Maureen Cropper, an environmental economist at the University of Maryland, notes, importing oil means exporting the damage associated with drilling for oil. When trying to put a price on that damage, do we think it varies by country? Is Kenya’s environment worth less than our own?

For all the complexity of calculating the true cost of gasoline, however, it’s unclear that it matters as much as some might think. When I started researching this column, my working assumption was that a world in which gasoline’s total costs were represented at the pump would be a world in which our consumption of gasoline was radically different. But almost all of the experts I spoke to said that wasn’t true. In part, that’s because years of regulations and innovations have made us much more efficient at finding, extracting, refining and using oil.

Stuck with oil?

If an energy source as dirty as coal had to pay its true cost, we’d pretty much stop using it. Disasters aside, that’s not the case with oil. Oil might be cheap compared with its true costs, but adding those costs in wouldn’t necessarily make it unaffordable. Our behavior might change at the margin, but we’d still be an oil-thirsty society.

That gets to the bigger issue, which is that energy sources are cheap or expensive only in relation to one another. And the heaviest anchor beneath our reliance on oil is that, at this point, there’s nothing to replace it with.

“We’re pretty much stuck with our dependency on oil,” Parry says. “We don’t have any substitutes. Even if we hugely increase the price on oil, we’d only have limited impact on it. People need to drive and get to work.”

That’s not to say there’d be no benefit to forcing gasoline to pay its full freight. Increasing the cost of oil could make other energy sources cheaper in comparison, and if the mechanism were a tax that would fund development of alternatives, that would hasten our transition. But it is the speed with which we can discover and refine those alternatives, more than the price of oil, that will decide our energy future.

The question, in other words, isn’t just what a gallon of gas costs. It’s what a gallon of anything that can replace gas costs. Maybe that’s what we should start asking politicians.

— Ezra Klein is a columnist for the Washington Post. His e-mail address is kleine@washpost.com.