Stocks have historically traded at an average of about 15 times yearly earnings. If company earnings simply compounded like interest each quarter, investors paying 15 times earnings would earn back their principal in just over 10 years. Those paying 21 times earnings, the current level for the S&P 500 index of large American companies, would earn back their principal in 15 years.

The companies below sell for more than 100 times earnings, for an earn-back period of at least 70 years. Why would investors pay so much? There are two broad reasons. They might believe that a company s future earnings will dwarf today s figure because the company is growing quickly, or because it s poised to recover quickly from a setback. Or, they might be so caught up in their excitement over a rising stock price that they stop caring about the mathematical details. (A former Federal Reserve chairman called that irrational exuberance.) All three of the companies below are projected to earn far more this year than last, and yet, even price-to-earnings ratios based on their forecast earnings look high.

Salesforce.com

Trailing price/earnings: 126
Forecast P/E (current fiscal year): 64

Salesforce.com makes software that companies can use to manage sales leads, customer service calls, marketing campaigns and more. Unlike database software installed on-site by industry giants like Siebel, Salesforce s software is cloud-based, meaning that users need only to log on through their web browsers. Companies pay a monthly fee per user for the service. Salesforce counts 72,000 mostly small companies as customers, and analysts say it s beginning to win contracts from larger customers, including Citigroup and Cisco. The company s newest application, called Chatter, is currently in beta testing and launches this year. Word has it, it s a bit like Facebook and Twitter for work.

Harley-Davidson

Trailing P/E: 110
Forecast P/E: 31

Last year s sharp contraction in consumer spending caused Harley-Davidson sales to plunge and its profit to shrink to a mere 30 cents a share, resulting in the lofty valuation. The question now is, what level of profit is normal for the company? Analysts expect it to earn 95 cents a share this year, against which shares would still look expensive. In 2006, the company earned nearly $4 a share; a return to that level would make today s stock price look low. Then again, 2006 marked the peak of America s house price bubble, during which homeowners cashed in equity to buy expensive toys, including motorcycles, so a return to $4 a share in earnings seems unrealistic in the near-term.

Wynn Resorts

Trailing P/E: 529
Forecast P/E: 111

Casino operator Wynn Resorts is based in Las Vegas but gathers two-thirds of its profit in the Monte Carlo of the Orient Macau, China. So although the Vegas economy is still hurting, Wynn is growing nicely on the strength of its China properties. However, Macau s government announced in March that it will tightly restrict casino expansion in coming years to keep development orderly. Surprisingly, Wynn shares have actually gained more than 15% since the announcement. Investors might be betting on a continued economic recovery in the U.S., but if so, they re betting aggressively. Wynn s adjusted earnings per share, currently below $1, peaked at just under $3 in 2007. Even a return to record earnings (which no analyst is predicting for Wynn this year or next) would leave the stock at about 30 times earnings. Then again, Wynn s earnings are held down by ongoing non-cash charges related to past investments. Ignore those and focus instead on the free cash the company is expected to produce, and share look a bit more sanely priced, if still not cheap. According to early estimates from BMO Capital Markets, an investment bank, Wynn should produce $4.34 a share in free cash by 2011.