Thinking about starting a listings website? Reader Mimi asks this week, "How do sites like Craigslist make money?"

It's no secret that most websites cash in through paid subscribers, advertisements, or some combination thereof. As a listings service, Craigslist makes its profit by charging users who post ads on their site. More specifically, Craigslist's primary source of revenue is paid job advertisements—$75 per ad in San Francisco, and $25 per ad in NYC, LA, Boston, Seattle, D.C., and other cities. They also charge $10 for paid broker apartment listing in NYC. While it may sound like peanuts, when you consider that the site gets around 2 million new job listings each month, those dollars add up. Because Craigslist isn't a public company, they don't have to disclose their financials. However, that hasn't stopped analysts from guessing at what the company rakes in. You can get a quick sense of that with a little simple math using the numbers above.

Actually generating profits is only part of Craigslist's success though. The company has managed to save a tremendous amount of cash by keeping their operating costs low. The site isn't fancy, doesn't require much maintenance, and almost never changes. The content is also created by users, and is largely moderated by them as well, although Craigslist management has taken a few steps to make sure people aren't committing crimes on the site. The majority of Craigslist's operating costs are probably a few servers—and paying upper management's salaries. Not bad, considering how much they're pulling in each month.

Beyond that, Craigslist has added value to itself by building a massive community of users. The site has an estimated nine billion page views per month, which makes it an incredibly attractive investment. That may explain why eBay is part of owner of the company (although that relationship's currently on the rocks).

That's the long answer to your question, Mimi. The quick and dirty is that Craigslist makes millions off a very simple model first developed by newspapers: charging people who want you to post their ad.