Farecast has been sold in a deal that values the Seattle online travel search startup at more than $75 million, according to a source with knowledge of the situation.

The buyer was not known and Farecast Chief Executive Hugh Crean declined to comment.

But one natural choice is Expedia, the Bellevue travel juggernaut that has acquired Seattle area startups in the past. A source in the online travel sector said rumors had emerged recently that Expedia was the buyer.

Expedia spokeswoman Katie Deines said the company does not comment on "rumors and speculation."

Last December, Farecast competitors SideStep and Kayak.com merged. At the time of that deal, Madrona Venture Group's Matt McIlwain, an investor in Farecast, said it could have "big, positive implications for Farecast." (In other words, a possible sale at a nice valuation.)

Farecast, which attempts to tell consumers whether airfares will rise or fall on selected routes, was founded by University of Washington computer scientist Oren Etzioni. It has raised about $20.6 million from Greylock Partners, Par Capital Management, Sutter Hill Ventures, WRF Capital and others. Recently, Farecast has expanded with a hotel offering and international airfare predictions. It launched domestic airfare predictions in June 2006.

The acquisition is the latest win for Madrona, which was one of the earliest backers of Farecast. Other portfolio companies that have sold recently include World Wide Packets, which was gobbled up by Ciena Corp. for $290 million. Last year, Bellevue's ShareBuilder was sold to ING for $220 million.

Interestingly, Madrona has some experience selling companies to Expedia. It was an early investor in Seattle-based VacationSpot.com which the online travel giant purchased in 2000.

More on Farecast -- which Time magazine called one of the 50 coolest Web sites of 2006 -- after it raised a $12.1 million venture round last year.

Stay tuned for more details.

UPDATE: Here's how Farecast compares to SideStep and Kayak.com in terms of visitors, according to Compete.com. What's interesting in the graphic below is that Kayak reportedly paid $200 million for SideStep, which at the time had about two million monthly visitors. (ComScore reports higher traffic numbers.)

That compares to Farecast's count of about one million visitors. If the $75 million figure is in the ballpark, that means the buyer is paying 75 times the monthly visitor count. Kayak, meanwhile, paid 100 times the visitor count at SideStep.

Both are pretty hefty price tags, since (as I have reported in the past) Google paid 23 times unique visitors for YouTube and News Corp. paid 7.3 times monthly visitors for MySpace.

Granted, Farecast and SideStep have much more targeted audiences in a consumer segment (online travel) where a lot of dollars change hands, so an argument could be made that their audiences are more valuable to advertisers.