The White House has told Congress that the massive influx of Cubans into south Florida could cost $300 million in the next four months, according to administration officials.

The estimate, in a letter sent to the Budget committees last week, would cover the expenses of the government agencies that have processed, housed and fed the nearly 75,000 newcomers as well as a full range of resettlement benefits comparable to those available to Indochinese refugees, one highly placed official said. It assumes that about 100,000 Cubans will arrive eventually.

The administration has been wrestling for weeks with the budgetary implications of how to treat the new arrivals and the message to Congress is the first indication of the likely costs.

The $300 million price tage is for the fiscal 1980 budget, which already is deep in deficit. Administration officials said they have no way of knowing yet what the Cubans' impact will be on the fiscal 1981 budget, which the White House and Congress have been struggling to balance.

A decision on how to handle the Cuban exiles may come by the end of this week, one official said. The main options seem to be narrowing down to a choice between using the emergency provisions of the new Refugee Act -- which would be more expensive since it calls for full federal reimbursement of state and local expenses -- or sending up special legislation to create a new category for the Cubans. This path could run into political problems in Congress from members concerned about the new tide of refugees.

So far, the administration has been careful not to call the Cubans "refugees" in the technical sense because of the impact. Instead, they are being processed as applicable for asylum.

But Jack Watson, the White House aide coordinating the logistical response to the flow of boats from Cuba, told reporters at a breakfast last week that it would be "impracticable" to process such applications case-by-case as the law calls for. That is why the special legislation route is being considered, he said.

While the policy planners struggle with the long-range impact, resettlement workers are at work trying to untangle a backlog of Cubans who have been processed but are unable to leave camps at Eglin Air Force Base, Fla., and Fort Chaffee, Ark., because of delayed security checks.

George Wagner, assistant director of refugee services for the U.S. Catholic Conference, which will resettle at least 70 percent of the Cubans, said Friday, that he had 3,000 sponsors -- mostly Cuban relatives -- ready to take people from Eglin. But the final clearances from federal authorities hadn't been received.

Wagner also expressed some concern about the ability of the volunteer agencies to find enough sponsors for all the single, unattached men who have showed up in the boat flow. "The singles will be more difficult to resettle, but we're not alarmed yet," he said.

At week's end, more than 31,000 of the Cubans had been temporarily resettled, most with relatives in the Miami area. Eglin was at its capacity of 10,000, but only about 1,050 had been resettled with the rest waiting.

Chaffee was close to its 20,000 capacity, with about 4,500 processed and awaiting sponsors and some 14,500 awaiting processing. Only 44 had been resettled.

Another 7,500 refugees had arrived at Fort Indiantown Gap, Pa., for processing, and federal authorities were setting up another center at Fort McCoy, Wis.

Meanwhile, a spokeswoman for the Federal Emergency Management Agency said yesterday in Miami that officials from Argentina had found 80 Cubans at Eglin who volunteered to resettle in that South American country. cOther Latin American countries and Australia have pledged to take small numbers of the refugees, she said.

Wagner noted that his organization has not received any federal money yet to offset the cost of resettling the Cubans because they haven't been declared refugees. The volunteer groups are getting $500 for each Indochinese they resettle.

He said he feels pressure with build on the administration to agree to a full benefits package for the Cubans, too.

One official involved in the policy deliberation acknowledged that the government is taking the chance of being "penny wise and pound foolish" in dealing with the Cubans. This is because studies have shown that money invested to aid refugees when they are pays off later by helping them get productive jobs.

There is some feeling in policymaking circles, officials said, that special legislation is attractive because many of the Cubans simply aren't real "refugees" facing persecution and might not need full benefits like the Vietnamese and Cambodians, because they have the support of relatives in the relatively properous Cuban-American community.

Declining to use the Refugee Act -- Carter's presidential challenger, Edward M. Kennedy (D-Mass.), has urged its use -- would make it easier to deal with the Haitians who have come to Florida by the thousands, in recent years, some officials feel. After a history of uneven treatment, the administration has pledged now to treat the thorize or appropriate millions of dol-Cubans and Haitians equally.

The problem with special legislation, though, is that there is no guarantee Congress is in the mood to aulars for a whole new category of new comers.

It's going to be a real tough call for us," one official said.