I don't blog all that often except on my mainstream blog. I got a post doing $100-$200 per pay period, but besides that I'm not seeing much in terms of consistent sales. I hear there are some high-profile mainstream bloggers making ~$15K/mo by selling ad-spots, to get to that I'd probably need a few thousand subscribers and learn how to write... I might see a good ROI if I paid someone else to blog, but right now the main reason I blog is just to get my thoughts out there. If my objective was to increase revenue, writing a blog post would be last on my list.
This guy (ok his blog is mainstream, but so what?), on the other hand, started monetizing his blog in Sep 2006 making $352.94/mo. In Jan 2007 he made $3440.66. Having 3895 subscribers (according to feedburner) doesn't hurt either. 3K/mo isn't a ton of money, but its probably more than enough to pay for the mortgage.
http://www.johnchow.com/making-money...-january-2007/