America's most expensive ZIP codes (© Radius/SuperStock)

Beverly Hills, Calif., ranks No. 5 on Forbes' list of most expensive ZIP codes. © Radius/SuperStock

Once, if a neighborhood made Forbes' list of America's 500 most expensive ZIP codes, it meant buyers were prepared to spend big bucks to call it home.

But in 2009, even in these exclusive enclaves, home prices took major hits. The ZIP codes on our list saw a 7% average drop in asking price, and in many places prices fell even more. Though Alpine, N.J. (07620), tops our list with a median asking price of $4.14 million, prices there fell 23% over the past year. Atherton, Calif. (94027), is the nation's second most expensive ZIP code, with a median asking price of $3.85 million, but prices there also declined by 23%. And New York's once-bohemian West Village neighborhood (10014) is by now fully gentrified, as demonstrated by its third-place finish and a $3.5 million median asking price. Still, over the past 12 months, prices in the West Village have fallen by 24%.

Overall, only one-fifth of America's most expensive ZIP codes saw prices rise — and in a few of those places, the data were skewed by a single high-priced listing. It’s safe to say that luxury home prices are down nearly across the board.

What's your home worth?

Take California, a state where housing speculation — and the subsequent market crash — peaked early. Last year, the Golden State accounted for 96% of America's 50 most expensive ZIP codes. This year, that figure dropped to less than 50%; 83% of those California ZIP codes that did make our list posted prices that declined or stayed the same.

But a few upscale neighborhoods are seeing prices climb again. New York's Upper West Side (10023) is one of them. Prices there have managed to inch up 4% in the past year.

"Higher-priced properties are coming on the market and staying on longer, so the stuff that is moving is lower-priced," says Michael Simonsen, CEO of Altos Research, a Mountain View, Calif.-based real-estate data firm.

The 5 most expensive ZIP codes

  1. Alpine, N.J. (07620)
  2. Atherton, Calif. (94027)
  3. New York (10014)
  4. Duarte, Calif. (91008)
  5. Beverly Hills, Calif. (90210)

Behind the numbers
Our list comes from real-estate statistics provided by Altos Research, a national real-estate data collection and research firm that tracks over 15,000 ZIP codes, amounting to about 90% of all real-estate transactions. Home prices are based on the asking price for combined single-family and multiple-family markets. ZIPs were ranked according to the median home price.

See video related to: fd22281a-7831-4ee4-bb2a-1d80d0de9f52
This video requires the Adobe® Flash® Player. Download a free version of the player.

Altos based its numbers on homes on the market as of Aug. 14, 2009, using Zone Improvement Plan codes as defined by the U.S. Postal Service. The USPS routinely adds new ZIP codes, in most cases due to shifts in population or in the volume of mail. Only a tiny percentage of new ZIPs affected our list of luxury neighborhoods, but those cases cause slightly uneven year-to-year comparisons.

Gun-shy buyers
The country's prime suburbs fared just as poorly as urban areas on our list. But unlike scores of lower-priced neighborhoods across the country, these areas are not necessarily experiencing rampant foreclosures or speculative flight. Rather, few potential buyers are biting.

Take for example affluent Atherton, where residents bring in an average annual salary of $122,571. Foreclosures don't explain its 23% price slide; only 10 homes are in foreclosure there. In picturesque Hastings-on-Hudson, N.Y. (10706), home prices have dropped 9% even though the village has no foreclosures in progress.

BingCrazy rich? Find luxury housing

Expect further declines before luxury home prices hit bottom. Buyers, spooked by the economy, are waiting for prices to fall even further. The problem is compounded by a lack of credit at the top end of the market.

Home affordability calculator

Potential buyers increasingly fall into one of two categories: those who have seen their net worth damaged by the financial crisis and those who realize they could probably get a better deal in a year's time.

Consider East Hampton, N.Y. (11937), which has been punished by declines on Wall Street. There are currently 765 homes on the market in East Hampton, but there have been only five sales in the last two months. At that rate, it will take 25 years before all the homes currently on the market (with a median asking price of $1.31 million) are sold.

The good news? Bargain hunters simply have to be patient, because sellers are soon going to have to offer hefty discounts.

View the rest of Forbes' 100 most expensive ZIP codes in America.

Additional reporting by Matthew Woolsey