I have just started reading this rather good blog in the last week or so. Daring Fireball is written and produced by John Gruber and covers design, technology and Apple geekery amongst other subject matter.
What struck me was the economics of producing a professional blog. The site is sponsored and advertising goes for $3,500 per week and sold exclusively to a single buyer. All but six of the weeks up until the end of August have been sold. I don't know what Gruber's track record is for selling his advertising, but if we assume he sells 40 weeks a year that's $140K.
Aside from that, Gruber runs a subscription service where members get exclusive access to RSS feeds of the site content. Membership costs a minimum of $19/year. Gruber states that he has over 150,000 members. I make that $285,000. So for 'simply' producing a great blog site we are looking at an estimated gross income of $425,000 per year.