There are a few ways you actually earn money from your blog. If you ever noticed that sometimes you have a few pennies here and there on some days and than non on others? This is because somebody left you a comment and depending on the blog the comment can be worth more. The higher the page rank of your blog, the more the comment is worth, although, I am unsure of the exact amount.
Another way that you earn money from your Google blog is, each time a reader follows your blog. This, of course, also depends on the page rank of your blog. If you blog does not have a rank of any type, then you will not earn and the lower the rank the less the money. Nobody is exactly sure of how much you earn since it probably depends on topics, advertisements and other Google terms of earning on your site.
Now you ask, how do I get a higher page rank? Well you need to receive comments and visitors to your blog. For every 1,000 actual visitors your page rank will go up, and for every so many comments left on your blog, your rank will also increase. Now how do you get visitors to your blog is the next question, social networking is one way. Unique topics is another way and if you can think of a topic before anybody else and it ages this will possibly give you leverage in being found via a Google search. Another way to earn is having interesting topics on your blog and receiving click through. Each time somebody clicks a link to another topic on your blog, you will earn and of course this depends on your blogs page rank.
Through out my blog are various topics on how I have received visitors to my blog, with a little hard work and reading of course, but it works. Everything here is free of charge so feel free to use anything and good luck!