IN 1981, when you have to be rich to afford a gutted rowhouse, it is reassuring to know that someone has been able to live even half as well as John D. Rockefeller Jr.

His name is Saul P. Steinberg -- the financier, not the cartoonist. His New York apartment is up for sale through Sotheby's for $10.5 million, the highest price ever in that city. The apartment is 39 rooms, one room less than half the size it was in 1929 when it was still possible to be very very rich -- and almost nobody was as rich as John D. Rockefeller.

Rockefeller built the apartment -- and the apartment building to hold it up, at 740 Park Avenue. The apartment -- for himself, his first wife (Abby Aldrich Rockefeller) and their five sons and a daughter -- then amounted to 90 rooms, the entire 14th through 17th floors. Of course, the Rockefellers only used it as a peid-a-terre. Their real home was in Pocantico Hills, on the Hudson in New York State.

According to Edward Lee Cave, chairman of Sotheby's International Realty Corp., "The concept of living in an apartment in 1929 seemed very strange to New Yorkers who where then accustomed to the mansion townhouses. But what the Rockefellers do is right, and they wanted a place high in the air, with excellent security. They set the style."

Martha Baird Rockefeller, Rockefeller's second wife, after his death sold off half the apartment, keeping the principal rooms. Then a decade ago, after her death, Steinberg bought her apartment for $285,000, according to real estate sources. He had the apartment elaborately decorated first by Sister Parrish, then by Stuart Greet. He put in new air conditioning and extensive security systems.

Now, perhaps because of an impending divorce, Steinberg is not only selling the grandiose apartment, but also the art collection that made the apartment a small museum. A total of 171 paintings and sculptures by 24 artists will be sold by Christie's in an evening sale tomorrow and a daytime sale Tuesday.

Some of the work has already been shown in Dusseldorf, West Germany. The art is expected to bring between $7.5 million and $9.5 million at Christie's auction. The remarkable collection includes many German Expressionist works, as well as others by Francis Bacon, Matisse, and bronzes by Rodin, Manuz and Barlach.

If the estimates are right, Steinberg should finish up with just under $20 million from the sale of the apartment and the collection.

It's not as if Steinberg didn't have income tucked into other velvet purses. The 41-year old financier is chairman of the Reliance Group (a Philadelphia insurance company), and owns a great deal of stock in The New York Times, Warner motion pictures, and the Rothschild Trust in London, to name a few. He made his first big splash with a computer time-leasing company called Leasco. He had explored the idea of time sharing on computers in his college thesis, which did not rate too high a mark, but in real life, he made a great deal of money from it, his friends say.

The $10.5 million price for Steinberg's apartment is shaking up even New Yorkers, heady from a year and a half of apartment boom. John Howard, head of Sotheby's real estate in Manhattan, says the nearest asking price to Steinberg's is an apartment listed at $7.5 million.

In Washington the 1930s Florentine villa on the old John Archbold Hillandale estate is for sale currently for $3 million from the Hillandale Development Corp. The mansion has just about the same square footage as in the Steinberg apartment but with an acre and a half of land. You can buy another four and a half acres of trees for $500,000.

The top price for an apartment in Washington, according to Elizabeth Twigg at Brennenman real estate, was the $1 million-plus former Gov. John Connally paid for Princess Shams' Shoreham West apartments. A Watergate apartment is currently offered at $750,000 and a Watergate pent house was sold recently for $585,000.

Not long ago, we went through the Steinberg apartment with Sotheby's Cave and Howard. Steinberg, according to reports, is avoiding the press after some stories in the New York newspapers about his planned divorce.

After coming through the Art Moderne lobby with its single doorman, we went up the elevator to the apartment's own lobby. We hardly had time to admire its Chinese furnishings, a reminder of the Rockefellers, before the burly security man answered the door. A security guard is on duty inside the apartment 24 hours a day. Daytime and nighttime butlers serve as well. Five of the staff live in. Two more come in by the day. Though it was 2 p.m., the maids were vacuuming the apartment, picking up clothes for the laundry and removing what looked like breakfast trays.

The gallery, as the reception room is called (56 by 13 feet), is actually the grandest and largest room in the apartment, the one that gives you a true sense of opulence. The other rooms of the apartment, if measured by, say, the British of Japanese embassy residences in Washington, might seem a bit cozy. Still, for an apartment, more than 15,000 square feet of space isn't bad.(An average efficiency might be only 400 square feet, to give you an idea.)

At one end was a splendid set of marble and ivory filigree furniture, from an Indian prince's home. Four ivory elephant tusks frame the set. "It's a cozy corner," said Cave. The floor is parquet in a diamond pattern. Some walls are covered in a velvet covering, with an ever so slight Art Moderne pattern.

On the gallery walls was a triptych, 6 1/2 feet high by 14 1/2 wide, painted by Francis Bacon in his usual grim, unpleasant if not obscene manner. This triptych and another by Bacon on the hall adjacent to the dining room are expected to bring so much money the prices aren't even estimated in Christie's hardbound sales catalogue.

A much more cheerful piece, an 84-inch bronze by Giacomo Manuz called "Cardinale Seduto," stood in the corner of the gallery. It's estimated to bring $150,000 to $200,000. Steinberg owns a similar marble statue by Manz, "Grande Cardinale in Marmo," which is expected to bring about the same price.

In the 23-by-40 foot drawing room, the most important piece was the overmantle around the fireplace, a painting commissioned in 1938 from Henri Matisse by Nelson A. Rockefeller. The painting hung in his New York apartment with its Art Deco furniture until after his death. The room was also full of paintings by A. von Jawlensky, as well as small sculptures by Rodin and Barlach.

Cave pointed out the secretary with its engraved ivory, as well as tortoise shell and ebony trim. The upholstered pieces are Art Moderne in feeling. French windows in the drawing room, as in other rooms, lead onto the terrace. The 12-foot-wide drawing room terrace is guarded by a Romanesque lion.

The 20-by-27 foot library is cozier with 1760 English pine paneling. A miniature secretary sitting on the floor is made of ivory in India, with fantasies of English houses cut into it. The chinese rug has the good fortune symbol used by the American Indians and the Nazis, among others. Max Beckmann paintings hung in the library and the next door billard room.

What is now the billiard room, according to Cave, was Rockefeller's Chinese retreat, where he displayed his famille noir porcelain. Some of the furniture from this room was given to the Vice President's residence in Washington by Nelson Rockefeller. Now the walls of the billiard room are a brilliant vinyl red. A handsome brass lamp with three shades hangs over the table. Adjacent to the billiard room, carved out of hall closets, is a bar; alternate walls, ceilings and floor are either black or mirror, making it a confusing, but dramatic space. A painting by Otto Dix hung over the bar.

The 23-by-33 foot dining room once held the great table seating 40 that Nelson Rockefeller gave to the Vice President's Residence, Joan Mondale was particularly fond of the table because it shrunk from banquet size to just big enough for the family. The room seats 48. The serving cart is ivory.

The paintings were more peaceful here, "Promenade" by Lionel Feininger and a flower still-life by Ernest Kirchner.

Steinberg displayed many of his Ernst Barlach bronzes in the dining room. Just in case Steinberg felt too lonesome in the drawing room, there's a smaller breakfast room, with an antique child's chair, probably used by his youngest child. He has three by his earlier marriage as well.

Instead of a first-floor powder room, a suite has coat closets as well for the ladies. The gents is down the hall off the billiard room.

The kitchen with its big restaurant range and a pantry as big as most people's living room is big enough to serve the untold multitudes, not only as many as could go around the table, but as many as could fill the hall. "Saul Steinberg likes to entertain -- dinners, theater parties," said Howard.The dishes are kept in a great number of glass-front cabinets. The silver goes in an immense safe adjacent to the kitchen. The servant's dining room is adjacent to the kitchen.On a lower mezzanine is a servants' area with three maid's rooms with two baths and a servants' hall.

On the second floor are five bedrooms, a study, two sitting rooms and a governess' suite with two bedrooms. There's a small kitchen up here as well, for serving breakfast. But the real charm of the second floor is the movie theater, where Steinberg's guests lounged in leather chairs while a touch of the button brought down a screen for the latest movies from Warner Brothers, et al. There's a gym up here as well.

What will the "vintner buy half so sweet as the stuff he sells?" Well, Sotheby's has this nice townhouse they'd like to show him. . .