(FORTUNE Magazine) – TOP FASHION DESIGNERS are discovering it's tough to live off the carriage trade in the automobile age. In order to grow they have had to reach down to the many who aspire to haute couture but don't have the money. Now more people can buy merchandise bearing fashionable names -- Yves Saint Laurent, Anne Klein, and Giorgio Armani among them -- with slightly less elegant tailoring and far less haute prices. The trick for designers is to widen their appeal without alienating loyal customers. The hottest prospective customers for designers who want to enlarge their audience are working women in the managerial ranks and nonworking women who emulate them. The five million American women earning over $25,000 a year will spend $18 billion in 1985 on apparel, predicts Isaac Lagnado, director of research at Associated Merchandising Corp., the consulting arm of a group of retailers with annual revenues of $10 billion. That's more than six times what women with comparable incomes spent in 1977, and roughly 13% of what all Americans spent last year on clothes and shoes. Among the most successful at capturing this new yuppie clientele is Anne Klein & Co., a New York fashion house owned by Japanese textile company Takihyo. The less expensive Anne Klein II line brought in $50 million in 1984, its first full year on the racks. This year the company expects sales of $75 million -- three times the revenues of its designer line. Who's buying? Apparently a woman confident enough to shed the dress-for-success uniform -- a masculine suit and bow tie -- and don a sexier broad-shouldered jacket and narrow skirt. ''She's elegant, feminine, and powerful,'' says Marilyn Kawakami, 42, president of the Anne Klein marketing subsidiary. She's also someone who can't resist prices 30% to 60% below those in the designer collection. Anne Klein II and other derivative lines save money by using cheaper labor and cheaper fabrics. They sometimes cut corners by leaving out linings, and they forgo the detail and handiwork that go into making designer clothes so expensive. A typical Anne Klein blazer costs $115 to make, wholesales for $200, and sells to the consumer for twice that amount. The Anne Klein II costs the company $45 and retails for $175. Fabric makes up a big chunk of the blazer's price. The designer version might use 2 1/4 yards of fine Italian wool at $22 a yard, while Anne Klein II settles for Oriental lambswool at $9 a yard. Workmen's wages on the designer jacket, made in the U.S., are roughly $53; on a cheaper one made in Hong Kong, labor costs run around $20. Designers who go too far afield in search of new sales risk losing their most important customers -- the moneyed followers of fashion who demand a degree of exclusivity. The greater risk, though, seems to be not catering sufficiently to yuppie tastes. ''Certain secondary lines, including ours, have not had enough of a career focus,'' says Richard Shapiro, executive vice president of Ralph Lauren Womenswear. Some designers turned out disparate items that didn't go well together; what working women want are outfits that are already fully coordinated. Maybe those seemingly composed yuppies aren't really so secure after all.