get glued to this nice forum and you will learn a lot that will ultimately enable you to earn while blogging...
If you're really asking how much, just take a look at this- http://www.johnchow.com/11-months-of-john-chow-dot-com-income-graphs/
It depends on how much you put into it and how much you do to promote it. Most money will come from links sales and blog posts and some from adsense and affilates. You have to get people there first to earn any real money.
You can earn money writing blog posts too. See my SIG - Smorty - I made $346 last month. Not earth shattering but helped!
You can earn more the better rank you have for: PR Alexa Technorati Backlinks # Pages Cached... Hope this helps a little more..
Sky is the limit. Jagsy- YPN ( If you are in the US), Adbrite, Kontera, Chitika, Valueclick, Casalemedia etc.
Here is a list of 15 blog advertising network. I think they are the best adsense alternative for bloggers. http://idigganything.blogspot.com/2007/09/get-paid-for-blogging.html If you have three or four page rank 4/5 blog (Not very popular) then you can earn almost $500 per month easily. John Chow earns $20,000 per month from a single blog and yaro starak $10,000. Best of luck. In future we will snatch their place. Haah?