Why buy that $2,250 Proenza Schouler party dress when you can rent it for a fraction of the price?

If Rent the Runway's new expansion is any indication, more and more women – who aren't ready to watch their style quotient plummet with their 401(k) in this tough economy -- seem to be asking that very question.

The Web site, which rents high-end designer clothing and accessories to its members, has been growing and growing since its launch last fall.

And, now, new multimillion-dollar investments are leading to more staff and bigger digs -- from a small shared office space in a dry cleaner's to a 9,000-square-foot headquarters, reports WWD.

The New York-based company has 500,000 members, with 20,000 new members signing up on average each week, according to the paper.

Here's how it works: Members pick a style from hundreds of top designers (think Badgley Mischka, Matthew Williamson, Hervé Léger, and Richard Chai), select a delivery date (a backup size will be sent at no extra charge), wear the dress or accessory to their event, and return it in a prepaid package -- the site takes care of cleaning costs.

So, a Brian Reyes gown that retails for $2,000 can be rented for just $250 -- and you don't have to make a trip to the mall.

Nearly 18,000 dresses have been rented since Rent the Runway launched, WWD reports, with an estimated $6 million in revenue in 2010 and more than $20 million in sales expected in 2011.

"We only buy current-season collections from our designers," Jennifer Fleiss, the company's co-founder president, says. "We go in just like any other buyers, and we pick a portfolio from that designer. Our buy differs from the department stores because we focus on the more editorial, embellished, fashion-forward, colorful styles. We don't pick the little black dress. Almost every dress on our site has something unique and special about it."

So, what's next for the site? Belts, scarves, shoes, and maternity clothes may be making their way into the mix.

Online shopping seems to be the stylish way to shop -- even eBay has a new Fashion app.