President Reagan's proposed space-based defense against nuclear missiles, if technically practical, would cost $770 billion to build and operate for 10 years, according to one of the first attempts to calculate the costs of "Star Wars."

Annual expenditures for such a system, according to a report released yesterday, would be "roughly comparable to what the Navy or Air Force currently invests in all weapons development and procurement."

The report, which makes several optimistic assumptions about technical progress in space weaponry, was prepared for the Johns Hopkins Foreign Policy Institute by two defense analysts and does not have government approval. The analysts, Barry M. Blechman and Victor A. Utgoff, acknowledge that their estimates are rough.

However, their study on the fiscal implications of Reagan's Strategic Defense Initiative (SDI) represents the most detailed effort to analyze in unclassified form how much a Star Wars system might cost. Lt. Gen. James A. Abrahamson, director of the SDI program, has declined to discuss costs, saying his research program is still too young to allow reasonable estimates.

"To some extent, the proponents have been unwilling to face up to the choices that would have to be made," Blechman said.

An arms control official in the Carter administration, Blechman added that by "injecting a note of realism into the debate," the study may prove "discouraging" to SDI backers.

The administration has said that a deployment decision on SDI can be made by the early 1990s. But Pentagon spokesman Robert B. Sims yesterday ridiculed any effort to calculate costs.

"We're not in a position to comment on cost estimates, no matter how exaggerated they may be," he said. "It is really an exercise in absurdity to try to estimate the cost of a system which hasn't been defined."

The report notes that the nation "clearly could afford to deploy" a Star Wars system if it chooses to do so. During peak building years, Star Wars would require an increase in the defense budget of $44 billion per year, equivalent to a $570 annual income tax increase for the average family, according to the report.

The SDI report analyzed several levels of strategic defense. For $160 billion, it said, the nation could build and operate for 10 years a ground-based antimissile system to protect U.S. missile silos, submarine ports and other military targets.

That $160 billion is equivalent to the cost of about eight aircraft carrier battle groups, the report said.

But building a population defense, which Reagan has said is his goal, would be far more expensive, the report said. The $770 billion, it said, would pay for ground-based missile interceptors, 9,570 decoy satellites and 1,900 battle satellites armed with antimissile rockets to shoot Soviet missiles as they take off.

In addition, the Western alliance would have to spend another $160 billion to improve non-nuclear weapons if the Soviet Union also builds a Star Wars shield that would blunt the usefulness of NATO's nuclear arms.

The estimate assumes that Star Wars weapons could be made 90 percent effective; a lower success rate would mean more would have to be deployed, increasing the cost. The report also assumes that satellites can be made resistant to attack without much added cost, a formidable technical challenge.

Utgoff, who works for a Pentagon think tank called the Institute for Defense Analyses, said he discussed the report with SDI officials who complained that the estimates do not allow for possible research breakthroughs. The report estimates, for example, that space-based interceptor rockets will cost $2 million each, while Abrahamson has said he believes they could be built for half that much.

SDI officials also have said that they hope to reduce space launch costs to one-tenth the current price. Utgoff said he believes such hopes are "whistling in the dark."