What does it take to get Donna Karan to drop $30,000 in less than an hour?

The answer, of course, is clothes – but not the Madison Avenue variety. We’re talking cast-offs at the Vintage Fashion & Antique Textile Show at the Metropolitan Art & Antiques Pavilion.

Watching Karan spend her money last Friday were some of Seventh Avenue’s most popular designers, including Jill Stuart, Betsey Johnson and Norma Kamali. Amid a crush of thousands the group roamed the aisles of the exhibition hall, on West 19th Street between Sixth and Seventh avenues.

It emerged that Karan was so keen to get first pickings she had asked Pavilion owner Alan Boss if she could come an hour and 15 minutes earlier than her peers.

Boss says she deserved the special treatment. ”Donna has been a very strong supporter of the event,” he told The Post. ”So when she calls and asks if she can come in at 10:45 instead of noon, we let her.”

Lucky Donna!

Lines started an hour before the Pavilion opened at noon on Friday. ”You have to get there early to get the best stuff,” says designing woman Stuart, whose addiction to vintage is reflected in many of her soft, youthful designs – and whose personal collection of 1,000 antique handbags grows each time she comes to the event, which is held four times a year.

The show features 45 specialized dealers of pristine vintage clothes, accessories, textiles and fashion-related books.

This past weekend the sale was visited by more than 1,800 antique-clothing lovers, including fashion magazine editors, supermodels and other designer fans of the show, who include Nicole Miller, Kenneth Cole, Pamela Dennis, Vera Wang, Stephen Sprouse and Vivienne Tam.

Vendors come from as close as Midtown to as far away as Amsterdam; shoppers travel even farther.

Designer Giorgio Armani recently flew a crew in from Italy to scour the show; Ralph Lauren and Calvin Klein also send undercover emissaries to scout for them.

It’s a rather morbid thought, but one that paints an accurate picture: Had a bomb gone off on West 19th Street this weekend, upcoming New York Fashion Week would have been a forlorn, lonely event.

What’s the appeal of this quarterly sale, now in its sixth year?

First, it’s the largest antique clothing show of its kind. Cruising the aisles of the 11,000-square-foot exhibit hall, you’re as likely to find fashion treasures from the 1700s as the 1970s, from highly collectible Gucci bags from the ’70s to beaded chiffon flapper dresses from the ’20s to rare Japanese obis and vintage corsets from the late 19th century.

The names on the labels are often as well-known as the names of the celebrities who buy them. For instance, you might see k.d. lang searching for Christian Dior, Bette Midler going in for Yves St. Laurent, or Marisa Tomei searching for Mary McFadden.

This year, five-time exhibitor Radford Brown, an owner of the downtown vintage boutique Cherry, sold Karan a pair of shiny white vinyl go-go boots from the ’60s as well as a ”fantastic” orange velvet and lace dress from the ’30s. He says that vintage couture pieces like these are always the first items to sell.

It’s true: Designers often visit this show to get ideas for new patterns. During the first 30 minutes of the show on Friday, Johnson, who proudly showed off a yellow appliqued pillow case, a tiny blue sweater with a felt flower pot sewn over one breast, and a celluloid hand fan, was overheard talking to Karan about a pattern maker who was an expert at crafting ”knock-offs.”

Designer interest in vintage clothing is not new. Anna Sui and Cynthia Rowley, who both shopped the show, are known to infuse their lines with designs borrowed from the past, whether they be fringed leather hippie jackets (Sui) or vintage Liberty print dresses (Rowley).

Shopper Marc Jacobs’ vintage fixation is evident in his spring ’99 collection, in which he borrows heavily from the practical chic of renowned American sportswear designer Claire McCardell.

But not all designers go in looking for goods to knock off stitch for stitch. Some are just shopping for inspiration.

”It’s like going to a museum,” says designer Randolph Duke, another fan of the Pavilion show. ”Occasionally you’ll find a garment with a single detail on it that’s so genius you can reinterpret it.”

The designer, who helped revitalize the Halston label before striking out on his own last year, says it was a glittering gown from the ’30s that he saw at a past Pavilion show that inspired the gem-dandy, racer-back number he whipped up for Golden Globe-winner Angelina Jolie to wear to the awards show a week ago.

On Friday, shoppers learned that later this year, the Pavilion will be tripling its space. By the October show (the next one is April 23 to 25, with another June 11 to 13), the new 33,000-square-foot area will be able to hold up to 70 dealers, Boss says, making the biggest show of its kind even bigger.

Happily, that means more vintage Pucci purses, more Gucci loafers and more Pringle cashmere sweater sets.

And more schmoozing.

”It’s a great place to socialize,” says Duke. ”Really, it’s the place to be seen.”