"HOW DO these artists make a living, anyhow?" asked Hirshhorn museum director Al Lerner, munching a homemade sandwich in the museum's executive dining room -- an automat in the basement.

He was querying Howard Fox, the Hirshhorn curator in charge of "Metaphor: New Projects by Contemporary Sculptors," the thought-provoking show that opened last week. Lerner knows well that all artists have trouble making a living, but he was especially baffled by the six in the Hirshhorn show, all former conceptualists and process artists now involved in making giant, one-shot works that cost thousands to build and almost never sell.

"Who would want them, where would they put them?" admits Dennis Oppenheim, who, along with Robert Morris and Vito Acconci, is among the best-known American avant-gardists now at work. Fame, however, doesn't seem to sell art of this particular size and weight -- both physical and intellectual. The Hirshhorn show includes three giant machinelike structures, one quiet room with poetry on the walls, another with massive black wood constructions and a stage set filled with skeletons riding warheads.

"No sculpture of this type is being bought," insists Oppenheim flatly. "You can't even give them away." He claims to have spent "more than $20,000" on the pyrotechnical machine he has made for the Hirshhorn show. Alice Aycock's fantasy in metal and glass cost "around $15,000," according to aides. Typically, a disheveled and unshaven Acconci had lost track of his costs, but was painfully aware that he owed the artists who were helping him over $7,000. "It's just awful," he said, "we all owe money."

The flat sum of $5,000 that the Hirshhorn contracted to pay each artist was nowhere near enough to pay all the costs of materials, shipping, labor and transportation here for the artists and their helpers. But there were no complaints. "Five thousand is generous by prevailing standards. We often get nothing," said Acconci, adding, "I lose money on every piece."

He has done better than most: He has sold three installations in the past eight years.

The answer to Lerner's question about how these artists make a living from their work would seem to be simple: They don't.

But how do they survive? "I sell a few drawings, but very few," says Oppenheim. "I lecture and do short-term teaching gigs and get an occasional grant," says Acconci.

"But the grant thing is wearing itself out. Many of us have already had two National Endowment grants and one Guggenheim, and that's all there is. And even the biggest grant, the Guggenheim, is hardly enough to pay for one of these installations. The only time I can work is when there's some money to start something, like this show."

"These are big pieces, and the only way to maneuver this work is to give it to a museum -- if they'll take it off your hands for a small fee," says Oppenheim. "Or you can attract somebody who will pay something for it and make a tax-deductible gift. We're obviously interested in perpetuation of the work, not extreme enrichment. All I'm concerned about is building the next piece. So if I can sell the raw material for what it cost me, I'm happy."

Among the other things that keep him happy are his frequent jaunts to Europe, where many small museums and collectors still clamor for advanced American work. They are often willing to pay transportation, expenses and -- sometimes -- a fee, in addition to offering the opportunity to install major works in public spaces. Acconci, Oppenheim and Aycock all have major European projects in the works for next summer, and life as a celebrity abroad, on what has come to be known as "the circuit," can be an agreeable substitute for riches.

TTHOUGH sums are small on those few occasions when money actually changes hands in the world of "project art," it begins, nonetheless, a remarkable kind of cash flow that might be called the trickle-down economics of the avant garde. The reason is plain. Sculpture exhibitions used to come in crates, and needed only to be put on pedestals. The installation projects in the Hirshhorn show had never been assembled before, and arrived -- mostly from New York -- in their most basic form in a convoy of rented trucks filled with lengths of wood, shaped pieces of galvanized steel and human skeletons painted black. They had to be built on the site.

As a result, earlier last week, the works of six well-known artists were being built at the Hirshhorn by an army of 30 unknown artist-assistants. Aycock, for instance, was present in spirit only as her fantasy machine was being put together.

Sculptor Andrew Ginzel, however, was there in the flesh, polishing large arcs of glass while Gregg Canright stood on a ladder, adjusting the wires that held them in place. Both are artists, but they were working for Aycock while she was in New York teaching an art class to earn the money to pay her assistants to build and assemble the works that have made her famous. "We do this to buy time to make our own art," the young artists explained.

There are also fringe benefits: Howard Fox expressed interest in the work of one artist and told him to send photographs; Joan Mondale took in three of Aycock's assistants as house guests. Fewer benefits accrued to Acconci and his band. Most, including Acconci, slept on the museum floor, and got a safety citation from Smithsonian security for cooking espresso on a hotplate placed on the gallery floor, presenting a fire hazard. Being sculptors, they found a sculptural solution: They put the hotplate and their battered espresso pot on top of a ladder and mounted the Smithsonian's violation notice below. Voila! A metaphor for the impact of overregulation in modern society.

JUST AS this art has required changes in its support system, so have economics brought about changes in the art. In the yet-to-be published catalog for this show, the Hirshhorn appears to have expected site-specific, temporary work that would cease to exist at the end of this show. Instead they got monumental works that are only vaguely related to the spaces they occupy. All could be easily adapted elsewhere.

The high cost of making these works has clearly led to a major shift. Even Acconci -- who, as a "body artist" in the '60s, bit himself all over and pulled out all his body hair -- admits he is seeking ways to make his work more permanent and portable. "I don't like the fact that pieces cost so much. It was far cheaper when I was using my own body."

So he has designed "Fan City" not only to last, but to fold up for storage, to be shown again. "I'd like to have the money to make it even more permanent, and install it outside," he says. At 41, even this former enfant terrible is beginning to think in terms of making art that that will outlast him.

As for Aycock's piece, it could be stored, but it is far too beautiful to be stashed away, and some museum may well buy it. Lauren Ewing has hopes of selling her "Auto-Plastique: The Prison" to a corporation with a big and relevant lobby. Siah Armajani has said he would like to give his "Hirshhorn Employees Lounge" to the museum, and Dennis Oppenheim is determined to reassemble his piece in an outside space in New York, where he hopes to shoot off the fireworks that were meant to propel it, but which were banned for the indoor showing here.

Only Robert Morris' "Journey of Death" has the sense of impermanence that has characterized most indoor project installations until now.

"It is ironic, I suppose," allows Acconci, "that these guerrilla fighters from the '60s are now beginning to make traditional studio art. But I've always had to adapt to time and space. Now I'm looking for permanence and portability."

But Acconci has not entirely let go his sense of the outrageous. Though his art has taken on some traditional sculptural forms -- his Hirshhorn piece is a carousel-like construction made of aluminum -- his interest in social interaction remains paramount.

"If the idea of making studio art bothers me too much," he concludes, "I can always think of myself as a guerrilla fighter making a bomb in his basement. And then setting it off someplace else.

"And you see? Even that's a metaphor."