I just wore a $500 dress to dinner. That’s about five times more expensive than the nicest thing I have in my closet. It wasn’t a present from a sugar daddy or a splurge after winning a hand of blackjack. It was a rental. And for $30, I felt myself striding through the Cosmopolitan on Valentine’s night like I owned the place.

That’s the idea behind Rent the Runway—getting high fashion into the hands of average girls who can’t afford “the perfect dress.” The web-based business has been so successful that it launched two showrooms in New York recently, and the pop-up that opened at the Cosmopolitan just before New Year’s Eve has been cranking out gowns, minis, wedding dresses and chic accessories by brands ranging from RVN and Reem Acra to Matthew Williamson.

Back in December, RTR co-founder Jenny Fleiss told me the Cosmo experiment would stay open through the end of March, but last week, assistant manager Jessica Stoller hinted the stay might be extended. Stoller was walking me through the in-store experience: “You’re paired with a stylist. … We’ll ask you what event you’re attending, what time of day it is, what your intentions are—like is your ex-boyfriend going to be there? Is your boss going to be there? That all reflects what you wear.”

Going through a hypothetical scenario involving a wedding (and the ex), Stoller and I covered basics like favorite silhouettes and colors down to my comfort levels with beading and bold lipstick. If I decided to wear my hair up, she’d show me styles that accentuate the back or shoulders. Stoller said she has a “Rolodex” in her head with brand details and size runs for every dress in the space.

I picked the RVN Night Racer (1) during an earlier visit to the showroom, but Stoller helped me scheme accessories for the glove-snug frock with a high neck, flared hemline and “lace tattoo detailing.” I was sure it needed a punch of color, a hot pink shoe or chunky green jewelry. Stoller gently suggested that with the dress’ cool aesthetic, that kind of accent would feel forced. And she was right. RTR’s “Go-To Girls” are style mavens who’ve tested many dresses themselves and read countless customer reviews, so they know what rides up and what photographs beautifully.

With my dress, Stoller preferred a black heel or bootie, maybe with a snakeskin-patterned clutch by Katherine Kwei (4) and a pop-art floral necklace from RTR line Slate & Willow (2). She also likes gold chandelier earrings by Lee Angel (3) that rent for $12. They’re current, like the dress, which is why Stoller steered me away from a vintage-style Oscar de la Renta necklace that would have netted RTR $85.

“It is way more important to get the customer looking good than getting them in any dress here ...” Stoller said of the lack of retail pressure. “I just make sure that whatever happens the customer is satisfied, that she’ll be thrilled with her dress and she’ll feel confident and good and get the Rent the Runway experience, that Cinderella experience where she’ll come in and be completely transformed.”

You can walk in unannounced, book a 30-minute appointment or reserve the whole shop so you and your friends can drink Champagne and have a dance party. Locals get 20 percent discounts on Mondays, which is great for clients like me. For those who have seven $1,500 Hervé Léger bandage dresses at home, it’s just as enjoyable to pop in for something fresh without the commitment. “It’s great for women who normally couldn’t afford the dress,” Stoller said, “and it’s also good for women who can afford the dress and just don’t want to wear it twice.”

Rent the Runway Through March 27, Monday-Wednesday, 11 a.m.-10 p.m.; Thursday-Saturday, 10 a.m.-11 p.m.; Sunday, noon-9 p.m. Cosmopolitan, 698-2500.