The Reagan administration is planning a major financial commitment to help rebuild Lebanon and is considering asking Congress for an aid package of up to a half billion dollars, according to a senior administration official.

The U.S. aid, envisioned as part of a wider international effort, will likely concentrate on helping the Lebanese rebuild roads, bridges, electrical and communications systems and water and sewer lines that were damaged in the Israeli invasion and in the preceding seven years of civil strife, U.S. officials said.

But the difficulty for State Department officials trying to determine how much the U.S. contribution should be is that there are no authoritative estimates of the scale of destruction.

Five months after the Israeli invasion, there are still no reliable casualty figures either. Casualty estimates have become weapons in the propaganda war that followed the bombing war.

The White House has a Lebanese estimate of 15,000 persons killed or wounded since the invasion, but little faith is put in those numbers if only for the reason that few here believe that the government of President Amin Gemayel, which has yet to extend its authority beyond the outskirts of Beirut, is able to make a systematic assessment.

International agencies have grown wary of making such estimates after sharp criticism by Israel and its U.S. supporters that earlier reports of carnage and destruction had been exaggerated.

"We make a definite effort not to be caught any more quoting any figures," said Michael Veuthey of the International Committee of the Red Cross. "Our principal purpose is to be helping people and not be used as a punching board by whomever."

"The figures are very invidious and very difficult to come by," a U.N. official said. "The Israeli government gives one estimate, the Lebanese give another. You have to agree that whenever you have an occupation, it's difficult to get an accurate figure."

U.S. officials have diligently avoided attempting to make any determination of the extent of casualties. They contend that the important task is to rebuild the war-torn nation, a project about which President Reagan is said to be personally enthusiastic.

A senior administration official estimates that the cost to the U.S. could run as high as $500 million although Senate sources said the figures the Agency for International Development has given Capitol Hill recently have been in the range of $150 million to $200 million.

But, another administration official said wearily, "State is mulling it daily. Their figures change daily."

The planning is going on as administration budget officials are struggling to contain spiraling U.S. budget deficits and at a time when other nations hard hit by the recession are looking to the United States for additional aid.

When Egyptian Foreign Minister Kamal Hassan Ali met with Reagan and senior administration foreign policy advisers last week, U.S. officials said, they got the impression that a request for a $400 million increase in military assistance for the next fiscal year was his most pressing concern.

A team from the World Bank is in Lebanon surveying the destruction to get an idea of what kind of international effort will be required after seven years of war to restore the once lovely country.

The Lebanese have estimated that the total public funds they need for reconstruction could be around $12 billion, but that estimate is not regarded as authoritative.

A group of American construction company executives who visited Lebanon recently, said for example, that the Lebanese were far too optimistic about being able to repair and rehabilitate buildings in downtown Beirut. It was the contractors' judgment that the buildings would all have to be razed because the cost for renovating them would be prohibitive.

Even before the Israeli invasion, the Lebanese had begun efforts to repair some of the civil-war damage, but reconstruction has been undone by the later fighting.

Mohammed Atallah, president of Lebanon's Council for Development and Reconstruction, lamented in a report last year on the progress of a four-year effort to restore Lebanon that the "rate of reconstruction was often surpassed by new destruction."

The Gemayel government hopes eventually to get aid to rebuild its crumbling infrastructure, not only from the United States but also from the World Bank, European nations and Arab states. It is expected that the rebuilding of homes and office buildings can be accomplished by private investment.

One big problem for the government of Lebanon is its inability to tap lucrative sources of revenue. Busy ports in the country, for example, are still controlled by the Christian militia and the Palestine Liberation Organization, who are presumably still pocketing customs levies, Lebanese sources here said.

Lebanon's banks are prosperous and have money to lend, but the American contractors who visited the country recently found that they are making only short-term loans. Like other potential private investors, the banks are waiting to see if stability can be achieved before making commitments for the kind of long-term loans that will be needed to rebuild the country.