The debate over whether the United States has tilted too far toward complex and expensive weapons rather than a cheaper, simpler and bigger arsenal began long before the Reagan administration took office and likely will persist into the next century.

In the early 1940s, a P51 propeller-driven fighter cost about $200,000. Today, an F15 jet costs $45 million. Part of that increase reflects inflation and part reflects the extraordinary increase in aircraft capability.

"It isn't as if you weren't getting anything for all this," said Russell Murray, a staff aide on the House Armed Services Committee. "Presumably, if you wanted to, you could go back and build [World War II] Spitfires and B17s. But I don't think anybody would want to fight that war.

"Some weapons are going to be very, very expensive. If they're very, very good, that's okay," he added.

From 1974 to 1981, the Defense Department bought an averge of 408 combat airplanes a year. From 1982 to 1985, the average has been 371 planes, according to the Committee for National Security.

Defense Secretary Caspar W. Weinberger says he believes that his firm hand has disciplined runaway costs. "I think the majority of defense system acquisitions are success stories," Weinberger wrote last year. "Unfortunately, these stories do not share the limelight with stories about the few problems that are uncovered."

Weinberger's reforms, including regular meetings with the managers of such large programs as the B1B bomber, have been aided by the sharp dip in inflation, which historically accounts for about one-third of Pentagon cost overruns.

Even so, most defense experts say they believe that at some point something will have to give. The nation will have to reduce the size of its armed forces, buy simpler weapons or find some other way to restrain arms costs.

No one has summarized the consequences of the current trend better than Norman R. Augustine, vice president of the Martin Marietta Corp., who predicted:

"In the year 2054, the entire defense budget will purchase just one tactical aircraft. This aircraft will have to be shared by the Air Force and Navy 3 1/2 days each per week, except for leap year, when it will be made available to the Marines for the extra day."