The annual Sports Illustrated swimsuit edition, which was started in 1964 as a means of filling space after the Super Bowl, has turned into a veritable cash cow for the publication.

In a recent Forbes story, reporter Patrick Rishe writes that the ad sales for this year's issue, released Tuesday, could be in the $40 million-$50 million range, if not higher. That's up from an estimated $35 million in 2005.

Editor Terry McDonell said in 2011 that the swimsuit issue generates about 7 percent of the magazine's annual revenue.

And the magazine isn't the only entity to take the swimsuit issue to the bank. Rishe writes that the models can also make a career out of appearing in the publication. Elle Macpherson, the last model to appear solo on consecutive covers, is now worth an estimated $45 million.

This year's cover girl, 20-year-old Kate Upton, will become the first model since Macpherson to make back-to-back solo cover appearances. And like Macpherson, Upton should be set for stardom. Upton, who has already signed endorsement deals with Mercedes, Skullcandy and SoBe, is reportedly worth $2 million. And seeing as how she is already having a positive effect on consumers, her endorsement profile is sure to take off.

The magazine will look to extend its readership this year with six pages of ads targeted at women. According to research conducted by the magazine, 18 million women flip through the swimsuit edition. That's nearly the entire audience for a normal magazine.

(H/T to Off the Bench)

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