Craigslist, the burgeoning online classified service, finally has a monetary value associated with the non-profit site: 100 Million.

The market research and consulting group AIM published a memo on the subject earlier this week in which they announced that Craigslist is on track to hit $100,000,000 in revenue this year—a 23% increase from last year. A notable achievement considering that newspaper classifieds have shrunk by a similar margin during the same time. AIM started tracking Craigslist in 2003, when they estimated the site was reaping around $7M / year in sales. Because the company is privately owned, official numbers aren't publicly available.

The site may have just proven it is possible to monetize online content that is generally expected to be free. By creating a service that is in great demand and patiently building its user base, Craigslist has apparently unlocked how to squeeze out some income: charge small amounts of money for discriminating types of classifieds while continuing to keep the main entree free. Craigslist allows people and companies to post free classified online ads in 570 cities around the world. But in 18 major American cities, the company charges recruiters $25 (or in San Francisco, $75) to place a help-wanted ad. In New York, it charges real estate brokers $10 to post individual listings for apartments and houses. Craigslist also charges for ads in its erotic services category, which the company recently renamed “adult services.”

Made infamous in recently months by a growing awareness of user safety and prostitution problems, Craigslist has had its hands full navigating negative press, skillfully sidestepping an agreement with state law-enforcement officials about ads for “erotic services,” fighting with part-owner EBay in court and staving off websites nipping at their heels such as IList.com, Backpage.com and Kijiji.com—which makes the 23% increase in revenue all the more impressive.

AIM concedes, however, that their valuation methodogy is no more complicated than counting all the Cragslists ads and adding them up. Which begs the question—how many interns it takes to count $100 Million worth of ads?