When a Paris couture evening gown costs as much as a house or a small yacht, all records in the history of high fashion are broken. France may be writhing through its austerity program, with the average person only allowed to take the equivalent of $350 out of the country during the whole year, but the couture collections for fall and winter have never been more opulent, luxurious, and inspirational.

It seems that down through the ages, every time France has been in trouble with wars, occupations, or civil unrest, designers have tended to counteract with totally flamboyant fashions. Such is the current situation, with at least five more years of Socialist rule looming over the horizon. Yet surprisingly, the couture turnover so far this year is up almost 40 percent over the same period in 1982. In the affluent crowd, the theory appears to be that if one has any sous left at all, why not spend them before the tax man grabs the lot.

Labor accounts for the major part of the astronomical costs of a custom-made gown. Karl Lagerfeld of Chloe fame, who designed his second haute-couture collection for the house of Chanel, showed several hand-embroidered evening gowns at mind-boggling prices in the vicinity of $40,000. Yet even if these dresses are never ordered by private clients (and even a millionairess would have to amortize such an investment by wearing it forever), the prototypes shown in the actual collections have given hundreds of hours of work to the skilled hands still able to create the fabulous embroideries that epitomized the 18th century, a period that Lagerfeld has always loved.

An internationally renowned fashion critic once observed: ''If the Paris couture did not exist, one would have to invent it.'' These biannual presentations continue to provide a veritable testing ground for trends that eventually take over throughout the world. They are loaded with ideas, many of which filter down to mass production and eventually affect us all. It's a constantly changing scene. Though this year's silhouettes do not outmode last year's, the newness is there, with subtle changes to study and adapt, even if it is only a question of an inch or two at the hemline or a new way of tying a scarf.

There are fabulous furs, but several designers, such as Jean Louis Scherrer, are reasonably realistic. They opt for false rather than true, with trimmings featured in the stenciled patterns of leopard, zebra, and giraffes - thus giving the endangered species the equivalent of a built-in life insurance policy.

Natural markings of the African animals also turn up in prints and Jacquard weaves, as well as super-modernistic embroideries; the latter are also treated in reptile designs. There are a few rather harrowing effects, with life-size snakes in shimmering sequins and beads twining round the body of skinny sheaths like a boa constrictor about to enjoy a good meal.

Geometry and asymmetry are leading trends. If you're contemplating something really new for winter or revamping an old favorite you already own, these ideas bear watching. Geometry may have been a big bore in school, but avant creators like Pierre Cardin have brought the slide rule into high fashion with color blocks, divisions, and asymmetrical cuts. Two overlapping revers may be set at one side contrasting ''nothing'' next door, or each lapel may step out in a different fabric - velvet on one side and satin on the other.

Deep-set batwing and Dolman sleeves often turn up in opposing colors, while Cardin's corolla and petal collars swoop off in winged effects at one side. Yves Saint Laurent's flattering little taffeta ascot scarves swathed high around the throat are pinned on the left shoulder with a huge glittering costume-jewelry clip.

The three big winners in Paris this season are definitely Yves Saint Laurent, Chanel, and Emanuel Ungaro. All offer vague hints of retrospection, yet are totally contemporary. In the midst of all the skin-tight, body-conscious silhouettes with broad padded shoulders, Saint Laurent brings back the unfitted, easy-to-wear chemise, often in the mood of the late Balenciaga, with straight seven-eighths-length tunics over a sliver of an underskirt in a contrasting color and fabric.

Emanuel Ungaro shows many of the most totally romantic evening gowns in Paris. Curiously enough, Ungaro, who started out as an assistant to the late Balenciaga, originally made his name in tailoring. The remarkable structuring of his coats and suits, his wizardry at mixing different fabrics and patterns in the same ensemble soon attracted an international clientele on the best-dressed list.

Today he is equally renowned for his ''soft touch'' - the Flou dresses and formal gowns that drew rave notices in his latest collection, based on fabulously intricate draping and pleating. Some of the slender Edwardian-inspired gowns with leg-o-mutton sleeves have little fishtail trains in sunray pleating, which the mannequins catch up and carry over one hand in the style of the belle epoque.

Everywhere the mood is one of supreme sophistication. Jules Francois Crahay of Lanvin dedicated his collection to the Duchess of Windsor. Jean Louis Scherrer, who dressed Mrs. Giscard d'Estaing when she was first lady of France, claims he has been inspired by Greta Garbo, though the idea occasionally appears somewhat farfetched.

Black is once again the top story on the palette, often relieved with flashes of brilliant color, especially after dark. Next comes the range of grays, looking very new in the pale-pearl and soft tonalities. Shimmering gray satins with a slightly iridescent hue, and gray lace embroidered with sequins or caviar beading evoke many of the sumptuous, luxurious effects that prevail throughout the couture.

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