Craigslist has always been murky about revenues, and remains a free site for most categories. It only charges a commercial rate for recruitment, with listings costing just $25 in 17 markets and $75 in San Francisco. Fees for apartment listings (in New York) and “adult and therapeutic services” ads fit into the nominal fee category, assessed mainly to track who is advertising and to limit ad spam.

Indeed, site leaders Jim Buckmaster and Craig Newmark have made a big point of saying they have enough money and don’t need to pump up the revenue machine — much to the frustration of eBay, which owns about 20 percent of the site.

But even with its modest feel levels, and mostly free status, the reality is that Craigslist is actually beginning to make real money, according to a new report and forecast by the AIM Group. AIM says Craigslist is poised to earn $122 million in 2010, with profits between $88 million and $99 million.

Recruitment accounts for more than half the site’s revenue, and roughly 17 percent comes from the New York-only apartment fees, says AIM. The big surprise is the projection for sex ads, which will bring in 30 percent of the gross. AIM notes that represents three times its projections for Craigslist sex ads in 2009.