Philippine President Ferdinand E. Marcos won't even get here until tomorrow, and already the red carpet his government has been rolling out for his first state visit in 16 years is estimated by critics to have cost his debt-ridden country more than $5 million. By the time he leaves the United States on Sept. 27, that figure could double, those critics assert.

To hear them tell it, it has been a summer-long spree of elaborate spending that probably hasn't been equaled since the shah of Iran, whose bank account was considerably fatter, used to come to town. The embassy has used picnics, luncheons, receptions, open houses, dinners and other mass social gatherings here and in five American cities to pave the way politically for Marcos and his wife, Imelda.

"How is that amount possible?" asked Rudolfo Severino yesterday when told of the multimillion-dollar estimate. Severino is one of a quartet of Philippine diplomats with ambassadorial rank assigned to the Washington embassy for the duration of the Marcos visit. "There are always some expenditures for receptions and luncheons. These are normal for an embassy. But I can't believe these figures."

According to critics, the expenditures have been far from normal, as the embassy invited members of the Filipino-American community to a constant round of activities in a effort to build a sympathetic constituency by the time the president arrived.

"There seems to be a kind of polarization going on here by the embassy," said David Valderrama, president of the Philippine Heritage Federation and of the Philippine Lawyers Association and a critic of Marcos. "For a country with people starving, if these figures are accurate, it's obscene to spend all this money, absolutely unconscionable."

According to Valderrama's sources, here's how the Marcos government spent its money:

* $410,000 for picnics in five U.S. cities;

* $60,000 for festivities surrounding the New York visit of retiring Julio Cardinal Rosales;

* $29,000 for an embassy luncheon series entertaining hotel, travel agency and congressional staffs;

* $52,500 for a daily embassy open house;

* $2,000 for seminars to discuss such controversial issues as martial law in the Philippines;

* $30,000 for twice-weekly embassy dinners;

* $695,000 for entertainers coming here within the last few weeks from the Philippines;

* $855,000 for public relations and government publications;

* $460,000 for ongoing embassy receptions;

* $2,575,000 for the presidential entourage, advance team and security.

Valderrama said "what really concerns me is how much money they've spent and the reaction to it by the Marcos opposition. I wish the Reagan administration would try to use its moral influence and help get everyone to sober up."