For most people, the answer is not so much.
JustinC of Neighborlogs recently wrote a blog post about how much he earns as a blogger writing about Capitol Hill. The breakdown -- about $14,000 a year. And that's posting quite a bit.
Another post at Problogger.net reports that while many people make no money at blogging, plenty of people make lots of money at it. Check out the details here.
PS: Did I mention Problogger.net? If you are seriously interested in step-by-step instructions on how to become a money-making blogger, go read everything ever posted there. They have lots of great ideas.
Sick of hearing about blogging yet? Tell me what else you want to know about, and I'll answer your questions with a post here.
Thanks for reading!