Writer's Digest cited the case of one blogger who supports himself with his online activity. David Rowell, "The Travel Insider," sends out his e-newsletter about the travel industry every week as well as maintaining a related website, www.thetrafficinsider.info. He doesn't charge for the information but once a year has a fund-raising drive during which he asks readers for contributions. His readership is around 20,000 but only about 1,000 of them send any money--an average of $50.
It's similar to the annual fund-raisers that NPR or PBS stations hold (although many of them have more than one a year). For it to work you need to have sufficient hard-core followers who value your information enough to pay for it even though there is a sea of free information out there as well. I don't know whether his ratio of 20 to 1 (that is, 19 readers who don't pay for every one who does) is typical, but it definitely can help to have a big pool of readers to start with.
It's not always about the numbers, though. Having a small pool of super-interested followers to whom you give exclusive or hard-to-get information can beat having a huge group who are only kind of interested and can get similar information elsewhere.
In other words, go big or go small--there's probably not much profit in the middle.
(My book, "Your Writing Coach," features two chapters about how to market your writing. You can find out more at www.yourwritingcoach.com or get the book from Amazon and other online and offline retailers.)