Manhattan condominiums have gained a reputation in recent years as a favorite store of wealth for the global mega-rich. But what if all those those Russian oligarchs, Chinese industrialists, and U.S. hedge fund managers would have been better off investing in a boring index fund instead than some of the nation's hottest luxury homes?

To see how the very top of condo market fared against less glamorous investments, the real-estate analysts at CityRealty looked at the average price per square foot across a collection of 100 prominent Manhattan condo buildings. The value of those condos increased 55 percent in the last decade, according to a new report, rising from an average of $1,530 per square foot in the second and third quarters of 2005 to an average of $2,371 in the same period this year. That was good for a compound annual growth rate of 4.5 percent—not strong enough to catch the 5.4 percent rise in the S&P 500 over the same period. Here's what the year-over-year increases look like.

That doesn’t mean Bill Ackman and Ken Griffin, to name two of Wall Street's bold-faced buyers of ultra-expensive pads, should have bought index funds instead. Diversification strategies aside, this analysis makes for an interesting but imperfect comparison. There’s no simple way to buy and sell shares in CityRealty’s basket of 100 luxury buildings, which include Manhattan’s most expensive and most exclusive redoubts. The condo data is based on a limited number of transactions and skewed higher by the trend towards building increasingly expensive apartments. On that note, the report suggests that the same hypothetical “index” of Manhattan's richest condo building will likely jump as super-elite towers like 432 Park generate enough sales to qualify.

And of course, if you buy a $100 million apartment for its investment potential, you also get the opportunity live in it. Or at least host the occasional cocktail party.