The Reagan administration told congressional leaders yesterday that the administration may seek $110 million in additional military aid for El Salvador, rather than the $60 million it had discussed earlier.

Most of the extra $50 million would be earmarked for rebuilding infrastructure like bridges and power lines, and for greatly expanded training of Salvadoran forces at Fort Bragg, N.C., in order to ease public and congressional concern that President Reagan might send large numbers of new American military advisers to El Salvador.

However, according to U.S. officials, bringing the Salvadorans to this country could cost as much as 10 times more than training them there.

The new aid approach was disclosed as leaders told Reagan that Congress probably will accede reluctantly to his request for more funds but only if he agrees to conditions.

These include ensuring that no U.S. military personnel are sent into combat situations and making a greater effort to work out a political and diplomatic solution to the Salvadoran conflict rather than seeking only a military victory.

"We will not Americanize this conflict," Reagan was quoted as saying aboard Air Force One as he flew to Orlando, Fla., yesterday, in an effort at reassurance.

In an apparent bow to the congressional concerns, he and other U.S. officials also took what sounded like a more flexible approach toward negotiations between the Salvadoran government and its rebel opponents.

But, while the term "dialogue" suddenly began appearing with frequency in administration pronouncements yesterday, Reagan made clear that he still opposes any talks that might give the guerrillas a share of power without their taking part in elections.

"I will not support negotiations that short-circuit the democratic process and carve up power behind people's backs," he said during his flight to Florida.

However, despite Reagan's stress on seeking a "common course" with Congress toward Central America, leaders of both parties who attended a White House meeting with the president and his top advisers earlier yesterday were put on notice that the price tag for consensus might be almost double what the administration originally asked for. Sources present at the meeting said the revised funds estimate, worked out by the administration over the weekend, was presented by Defense Secretary Caspar W. Weinberger.

The sources said the president stressed that he has not yet decided on a specific amount. It was also revealed at the meeting that the administration is considering asking for an economic aid package that could be $120 million or more.

White House officials, in talking about the meeting later, gave no indication that the administration plans to go beyond the $60 million it originally said was necessary to maintain the level of U.S. support for the Salvadoran government's campaign.

However, the sources added, Weinberger gave a detailed breakdown of the estimated Salvadoran needs that added up to $110 million. They said he described the bulk of the money as necessary for training and added that, in other major categories of assistance, the administration wants to provide $15 million for ammunition and $35 million for "consumable" supplies.

The president began describing the situation in crisis terms about a week ago, after Congress failed to pass a foreign aid bill and instead adopted a continuing resolution that gave the administration $26 million for the present fiscal year rather than the $61 million it had sought.

At a meeting last week Reagan told Congress that if the administration was unable to come up with $60 million to cover that shortfall the Salvadoran forces would be in danger of running out of ammunition.

Immediately afterward, a senior White House official created alarm and controversy by revealing that the administration was considering increasing the number of advisers--now at an administration-imposed limit of no more than 55--and changing their role, which had been limited strictly to training duties outside combat areas. Several of those who attended yesterday's meeting said it seems certain that Reagan will ask Congress to provide the funds in the form of a supplemental appropriation.

The administration does not want to ask for a reprogramming because that would take aid money away from other countries, and Congress is hostile to the alternative of bypassing the legislative process and taking the money out of a special emergency fund at the president's disposal.

One of those present, Sen. Charles H. Percy (R-Ill.), chairman of the Senate Foreign Relations Committee, said he believed the president will get the money, but with conditions attached.

Percy said they must include an emphasis on long-term economic aid as well as military help, an amnesty program for the guerrillas and provisions to allow them to take part in elections safely, a "serious conversation" with the opposition, as advocated by Pope John Paul II in his visit to El Salvador last weekend and a reinstitution of the criminal justice system to curb human rights abuses and prosecute the killers of several Americans.

On the flight to Florida, White House press spokesman Larry Speakes said Reagan has six principles governing his policy in Central America.

Some appeared to coincide with points made by Percy, but Speakes also quoted the president as stressing his determination to deter the Soviets and Cubans from aiding guerrillas through Nicaragua and to support talks between Central American countries to resolve regional conflicts.

State Department spokesman John Hughes, expanding on the "dialogue" theme, said the administration previously had "shied away from the term" because it was regarded as implying power sharing.

But, he added, "We support a dialogue on how to participate in the elections and the evolving democratic institutions."