A Senate subcommittee held a hearing yesterday on the modern American equivalent of the ancient Egyptian pyramid -- the presidental library -- and other costly perks of "an imperial former presidency."

"This year, for the first time, the amounts we will spend on former presidents will be more than the appropriation of the [present] White House," Sen. Lawton Chiles (D-Fla.) said as he opened the first of three days on the subject.

U.S. taxpayers will pay an estimated $18.3 million to memorialize protect and maintain offices for former presidents in 1980, nearly 300 times as much as in 1955, he said.

The cost of Secret Service protection for former presidents, their wives and widows has risen from about $50,000 in 1964, when it was initiated, to almost $8 million this year. Six persons are receiving this protection -- former presidents Richard Nixon and Gerald Ford and their wives, and the widow of presidents Truman and Johnson.

And the cost of office staffs has grown from $160,000 in 1959 to almost $800,000 this year. Chiles said he considers tis "remarkable" because private earnings by former presidents hve also risen and because they "have no official duties or responsibilities . . . to perform in exchange."

It is not only rising costs, but also expanded egos, larger buildings and the paper profligacy of the Xerox era that have sent these costs skyrocketing, yesterday's testimony indicated.

For former president Ford, there will be two facilities to maintain -- a Ford Library that is being constructed in Ann Arbor, Mich., and Ford Museum being built in Grand Rapids, more than 100 miles away, an official of the General Accounting Office noted.

Under the law, the federal landlord agency, General Services Administration (GAS), is authorized to accept donated buildings and presidential materials, and maintains them with tax dollars.

However, in some cases GSA also has paid to expand existing buildings, according to testimony yesterday. $ the cost to the taxpayers for six existing presidential libraries -- those of Hoover, Roosevelt, Truman, Eisenhower, Johnson and Kennedy -- is expected to total $9.5 million in 1980, by GSA estimates.

When the original law was passed, income from fees or donations was expected to offset about one-third of the libraries' annual costs, GAO's Donald L. Eirich said. But this year, such income will defray only about 10 percent of the cost.

At the request of the Johnson family, for example, the government agreed not to charge visitors to the Johnson Library, though other presidential libraries charge 75 cents admission, Eirich said. This revelation raised Chiles' eyebrows.

"It's hardly an arm's-length transaction," he said, "when you have a director of GSA forced to negotiate what the deal will be with his own boss," the sitting president.

"Lyndon Johnson always did like big crowds, and he'd do anything he could to get one," Sen. David Pryor (D-Ark.) added drily.

The libraries were intended mainly as resources for scholarly research, Eirich said, but to appeal to the public have turned increasingly into museums.

Last year, only 1,100 visitors to the facilities, less than 1 percent, were researchers.

The senators noted that a library for former president Richard Nixon is still in the talking stages because his presidential papers have been tied up in court, and no cost estimates are available.

Several early presidents, Madison, Monroe and Jefferson among them, died penniless, Chiles said. But thanks to the sale of memoirs, exclusive television interviews and other ventures for modern presidents, he added, "the pendulum seems to have swung to the other extreme."