Of course your results will vary, but generally speaking, the "click through" percentage (people clicking on ads) will be less than 1% of visitors, and while some specific ads do carry higher payloads, most will bring you less than fifty cents in revenue. So, it's simple math: if you have 100 visitors a month, you might not even get one click, and if you do, it might not earn you enough to pay for a cup of coffee.
With optimization, you might get the CTR a little higher, but it should be obvious that it's never going to be very high: this is a volume game and you need volume to make money. Certainly there are exceptions: if your blog focus is in certain subject areas where ads are highly competitive and pay very well, you might get decent income from low volume. But for most blogs, the ads will be low paying so the only path to real money is increased volume. Don't get totally discouraged by that, but it is reality for most sites.
Now, Google would get very upset with me if I told you what my main site earns, but you can probably get a good idea from looking at the site stats and doing a little guesstimating. If you aren't horribly math challenged, you can see that even a fairly active site like this isn't piling up enough income to live on. Not that I'm complaining: what does come in is very welcome. But I'd need five times the volume for this to be anything like a real income (I used to say ten times, but it does keep growing)
source: http://ads-template-pro.blogspot.com