The General Accounting Office has prepared a draft report now on President Reagan's desk that casts serious doubt on the effectiveness of the administration's much-publicized war on illegal drugs.

Since the administration began its assault on the illegal drug trade a year ago by setting up a federal drug task force in south Florida, prices for major drugs have actually dropped slightly nationwide, indicating greater availability, despite the task force's efforts, according to the report.

The price of heroin fell from $2.25 a milligram in 1979 to $1.66 in June, 1982, while cocaine declined from 65 to 52 cents. The price of marijuana has dropped from its recent high of $1.38 per gram in 1980 to $1.32.

The report, a copy of which was obtained by The Washington Post, also cited other problems encountered by the task force, although many of them existed long before the task force was formed.

These included lack of coordination and antagonisms among the participating agencies, failure to prosecute drug traffickers once they are arrested, and two agencies each counting the same arrest.

"Several DEA Drug Enforcement Administration and other agency officials told us that even though the task force has caused many traffickers to curtail or move their smuggling operations, it is doubtful whether the task force can have any substantial long-term impact on drug availability," the report said.

The report, which is seriously critical of federal drug-enforcement accomplishments over the past several years, also estimated the 11-month cost of the south Florida operation at $66 million, far higher than administration estimates.

Last Oct. 14, President Reagan declared war on drug trafficking and announced plans to set up 12 additional task forces, modeled after the one in south Florida, to cover the entire country.

The total annual appropriation for those task forces, which were scheduled to begin a phased-in operation this week, is $127.5 million.

Sen. Joseph R. Biden Jr. (D-Del.) presented the report yesterday to the president in a meeting to discuss a recently passed crime bill now awaiting the president's signature.

Art Brill, a spokesman for the Justice Department, said last night that the department received a copy of the draft report Dec. 15 and has distributed it to agencies within Justice for comment.

Brill warned that draft reports may have "errors of fact" and that none of the key officials at Justice has had time to respond.

He said the department would have no official comment on the GAO document.

In a speech in Miami in November, Reagan told a citizens' group, "There's no question that the South Florida Task Force has been a clear and unqualified success. Since its inception, drug-related arrests in the area covered by the task force are up 27 percent.

"Drug seizures are up about 50 percent . . . . The amount of marijuana seized has increased by 35 percent, the amount of cocaine by 56 percent."

According to the GAO report, DEA figures show that only 5 percent of the defendants arrested by the task force are considered major violators.

"Double counting of drug seizures makes it impossible to get an accurate count of the drugs seized," the report said.

"The largest cocaine seizure in history--3,245 pounds--which was made in March, 1982 at Miami International Airport, was counted by both DEA and Customs."

The GAO report also said that some of the agencies working on the task force, including the Drug Enforcement Administration, the FBI, Customs, the Bureau of Alcohol, Tobacco and Firearms and the Coast Guard were antagonistic toward one another.

The report said that 85 percent of the people arrested by the Coast Guard and Customs between 1977 and June, 1982, were not prosecuted in federal courts, and no data were maintained to find out what happened to them.

One reason is believed to be the longstanding animosity between the law enforcement agencies such as DEA and the FBI, which began last year to work cooperatively on drug cases, and agencies such as the Coast Guard and Customs, which intercept drug shipments but are expressly forbidden from taking part in drug investigations.

Traditionally, DEA has concentrated on its own cases, which often involve higher-level criminals, and tended to give less emphasis to the others.