Time and Money
Joanne Jacobs on bloggers balancing free and pay. Even a few free posts are enought to give readers a taste of a writer's work, as Amazon's "look inside" scheme demonstrates for books. The full blog, timely and comprehensive, is worth real money in its own right.
Of course, bloggers can also promote their other writing and books on Blogging Network. One advantage: they know the readers are willing to pay for quality writing.