Stopping illegal drugs at the border may be a more cost-effective way to take drugs off the market than law enforcement inside the United States, according to a study done for the U.S. Customs Service.

The study by the Wharton Econometrics research group said that every dollar spent last year on intercepting drug shipments before they entered the country resulted in the seizure of $7.05 of cocaine and marijuana.

By comparison, the study said, every dollar spent on drug investigations inside the country turned up $3.37 of cocaine and marijuana.

Nonetheless, the bulk of the money spent on drug enforcement last year went to investigations rather than interdiction. The Customs Service, a branch of the Treasury Department, is responsible for preventing the illegal flow of drugs into the country.

"The policy implications are that if drug removals have a large impact on the market, then the efforts should be concentrated at the interdiction stage where large amounts of drugs can be seized for less law enforcement effort," the study said.

The study, based on responses to 1,161 questionnaires sent to law enforcement agencies, estimated federal, state and local efforts for enforcement at $6.2 billion in 1986. Of that, $5.3 billion was spent on investigations, with the rest devoted to federal interdiction, the study said.

The expenditures resulted in seizures of more than 127,000 kilograms of cocaine and more than 3.5 million kilograms of marijuana, the study said. The Customs Service says the figure had never before been compiled.

The 136-page report found that the funds spent by state and local police to combat illegal drugs represented 18.2 percent of their total budgets. Spending in Florida, where the drug problem is the most critical, totaled $450 million, which is more than 9 percent of the $4.9 billion spent at the state and local level nationwide on drug enforcement.