Of all the statistics that President Obama’s national security team will consider when it debates the size of forthcoming troop reductions in Afghanistan, the most influential number probably will not be how many insurgents have been killed or the amount of territory wrested from the Taliban, according to aides to those who will participate.

It will be the cost of the war.

The U.S. military is on track to spend $113 billion on its operations in Afghanistan this fiscal year, and it is seeking $107 billion for the next. To many of the president’s civilian advisers, that price is too high, given a wide federal budget gap that will require further cuts to domestic programs and increased deficit spending. Growing doubts about the need for such a broad nation-building mission there in the wake of Osama bin Laden’s death have only sharpened that view.

“Where we’re at right now is simply not sustainable,” said one senior administration official, who, like several others interviewed for this article, spoke on the condition of anonymity to discuss internal policy deliberations.

Civilian advisers, who do not want to be seen as unwilling to pay for the war, are expected to frame their cost concerns in questions about the breadth of U.S. operations — arguing that the troop surge Obama authorized in 2009 has achieved many of its goals — instead of directly tackling money matters. When the president’s war cabinet evaluates troop-withdrawal options in the next few weeks presented by Gen. David H. Petraeus, the top coalition commander, “it’s not like each of them will have price tags next to them,” the official said. But “it’s certainly going to shape how most of the civilians look at this.”

The question of cost will have a far greater impact on the eventual decision than it did during the White House debate about the Afghan surge in late 2009. The heightened fiscal pressures, coupled with bin Laden’s killing four weeks ago, could shift the balance of power in the Situation Room toward Vice President Biden and other civilians who had been skeptical of the surge and favor a faster troop drawdown than top commanders would prefer.

“Money is the new 800-pound gorilla,” said another senior administration official involved in Afghanistan policy, who also spoke on the condition of anonymity. “It shifts the debate from ‘Is the strategy working?’ to ‘Can we afford this?’ And when you view it that way, the scope of the mission that we have now is far, far less defensible.”

Military and civilian officials agree that the cost of the Afghan mission is staggering. The amount per deployed service member in Afghanistan, which the administration estimates at $1 million per year, is significantly higher than it was in Iraq because fuel and other supplies must be trucked into the landlocked nation, often through circuitous routes. Bases, meanwhile, have to be built from scratch.

The U.S.-led effort to create a new national army, which Afghanistan never had, already has consumed more than $28 billion. The Pentagon wants $12.8 billion for fiscal 2012 — the largest single line item in next year’s Defense Department budget request — to continue training and equipping Afghan soldiers.

To civilian administration officials, the budgetary drain of the Afghan war means fewer resources to put toward other pressing national security challenges.

Last year, the United States spent nearly $1.3 billion on military and civilian reconstruction operations in one district of Helmand province — home to 80,000 people who live mostly in mud-brick compounds — about as much as it provided to Egypt in military assistance.

Civilian officials expect top military commanders to resist calls for steep reductions. Military leaders maintain that the 30,000-troop surge and an increase in civilian reconstruction efforts have resulted in a dramatic turnaround of what had been a foundering war, creating the possibility of a reasonably stable nation.

They insist that a rapid withdrawal of forces would make that goal unachievable by rolling back territorial gains against the Taliban and jeopardizing efforts to develop Afghan security forces and build government institutions. U.S. military officers also contend that the aim of a negotiated settlement with the Taliban — an outcome espoused by the White House and the State Department, but not as vigorously embraced by top commanders — would be at risk if there were fewer troops to pressure the insurgents.

“We’re at a critical point in the war,” one senior military official said. “If we send the message that we’re letting up, what incentive does the Taliban have to make a deal with us?”

Civilian officials argue that recent gains against the Taliban and al-Qaeda have largely been the result of a counterterrorism strategy implemented by Special Operations forces, not the costly, large-footprint counterinsurgency mission that aims to secure the country district by district. Reducing conventional forces, some civilians assert, will not fundamentally alter the calculus that has led to interest among Taliban leaders in exploring peace talks with the Afghan government and U.S. representatives.

“Our mission is to disrupt and dismantle al-Qaeda, and what the bin Laden killing shows us is that you can do that with a small number of highly skilled guys,” the second senior official said. “You don’t need Army and Marine battalions in dozens of districts.”

Concern about war costs is putting new political pressure on Obama, much of it from fellow Democrats. On Thursday, the House narrowly defeated an amendment calling for an accelerated withdrawal from Afghanistan and a fixed timetable for turning over military operations to the Kabul government. The vote, 204 to 215, was far thinner than last year’s 162-to-260 tally on the same issue.

In the Senate, influential members have said recently that the cost of the war merits a reexamination of the overall U.S. strategy in Afghanistan. “It is fundamentally unsustainable to continue spending $10 billion a month on a massive military operation with no end in sight,” Sen. John F. Kerry (D-Mass.), the chairman of the Foreign Relations Committee, said this month.

Some Republican presidential hopefuls also are beginning to have second thoughts about the scope of the war, which White House officials think could provide political cover to Obama as he pursues a drawdown. Among those who have questioned the cost is former Utah governor Jon Huntsman Jr., who told ABC News that “we have to evaluate very carefully our presence in Afghanistan,” which he called “heavy and very expensive.”

An initial indication of the White House’s view on the costs occurred this month when the National Security Council rejected the military’s request to expand Afghanistan’s security forces by 73,000 personnel.

Concerned not just about the price of training but also the cost of maintaining the force — estimated at $6 billion to $8 billion a year, which far exceeds the resources of the Kabul government, whose annual budget is about $1.5 billion — the NSC authorized the addition of just 47,000 personnel. That would bring the total combined size of the Afghan army and national police force to 352,000.

“We’re building an army that they’ll never be able to pay for, which means we’re going to have to pay for it for years and years to come,” the first official said.

Military officials said reducing troop levels might not reduce costs proportionally because of the need to sustain bases and other infrastructure. Their intention is to “thin out” U.S. forces in many areas, not withdraw entirely, to facilitate an orderly transition to the Afghan government. “Pulling out more forces than prudent may not yield the cost savings everyone wants,” the senior military official said.

Although troop reductions will almost certainly begin in July — the month Obama promised to start a drawdown — military engineers and contractors continue to expand bases across southern Afghanistan.

At Camp Leatherneck, the main Marine outpost in Helmand province, workers recently finished building a second runway that can accommodate the Air Force’s largest cargo jet, even though some military officials deemed the existing runway sufficient. The base also has been outfitted with paved streets, complete with American-style signs.

Recent supplemental appropriations to fund the war, which have included billions of dollars for construction and equipment, “have been like crack” cocaine for the military, said one officer in southern Afghanistan.“We’ve become addicted to building.”

But moving too aggressively to control that spending could open the White House to criticism that it is depriving troops of necessary supplies and infrastructure. As a consequence, administration officials have concluded that the only practical way for them to bring down costs is by reducing troops.

“The head count is the only variable that we can control,” said a civilian official involved in war policy.