Blogs can be massive money makers if you play your cards right. Basically, your blog's income will depend on two things: how "sellable" your topic is (as that will determine your audience) and how many people visit your blog every day. TechCrunch, for example, pulls in $60,000 a month off advertising. I should note on that point, though, that a lot of bigger blogs prefer CPM models over CPC ones.
If you're looking for ways to make money from your blog, check out Darren's
http://www.problogger.net - it's an amazing site for any aspiring blogger.