And then there's the price tag: $7,000 at Main Street Coin's gallery in Fairfield, Ohio.

"You buy coins because you love them," says Mike Dickmann, gallery co-owner. "For 40 bucks, you can buy history."

In fact, the market for investing in rare coins has been building since 1999, when the U.S. Mint sparked new interest in coin collectibles by issuing the first state commemorative quarters. The U.S. Mint has sold 24.71 billion of them.

Other factors sparking the boom, experts say, are the Internet's impact on sales and knowledge, recent low interest rates and a loosening of once-strict grading values for coins.

This month, a rare 1913 Liberty Head nickel sold for $4.15 million in New Hampshire. In April, the world gold coin collection of the late Louis Eliasberg, a Baltimore financier, set several records when it was auctioned in New York for more than $10 million.

At least 300 rare coins have sold for more than $200,000 each, leaving general investors calculating coin return rates in the face of, for example, the 200-point dip the S&P has seen in five years.

Values for virtually all series and grades of U.S. coins have risen substantially over the past three years, according to Mark Ferguson, a market analyst for Coin Values magazine.

And while Ohio apparently is the only state to ever invest in rare coins, that could change: The state's original $50 million investment had yielded a $13 million return since 1998, according to state records.

But coins are not an investment tool for novices because of their complexity, political nature and cyclical value, warn experts such as Jeff Garrett of Lexington, Ky.

Garrett, author of "100 Greatest U.S. Coins," is working on the Encyclopedia of United States Gold Coins in conjunction with the Smithsonian Institution.

"I won't work with anybody who just wants to invest in coins," Garrett says. "Whether it's fine art or whatever, the joy of it is to become a passionate collector. ... It's not like investing in Microsoft."

Paul Burns, 43, a Hamilton, Ohio, man who was motivated to collect coins by the Mint's new state quarters program, has more than 1,000 coins, most of them from the 20th century. His Benjamin Franklin half-dollars, for example, were minted from 1948 to 1963. But they are just a piece of his investment portfolio, which includes a 401(k), savings and a vacation home.

Tim Hunter, a Fairfield, Ohio, collector, equates coin investing with buying a house. "If you are careful and selective, then your house will appreciate" in value, he says.

To fill out a series of French francs he was collecting, Hunter paid $1,500 at a Hong Kong auction for a 1914C franc, graded MS 65, or above average.

"It's unobtainable now," says Hunter, a retired high-school principal who teaches composition at Miami University in Oxford, Ohio. "I could get double or triple that price. But I'm not selling. Yet."