How much traffic do you need to make money with Google AdSense?

Recently we have built quite a few sites that offer freeware software or free content on a subject matter with an advertiser monetized publishing model and the majority of these clients are looking to AdSense as their revenue source.  For those of you that don't know Google AdSense is:

"Google Ad Sense As defined by Google- An ad serving application run by Google Inc. Website owners can enroll in this program to enable text, image, and video advertisements on their websites. These advertisements are administered by Google and generate revenue on either a per-click or per-impression basis"

One question I often here is:

How much money will I make with AdSense?

With our clients permission we took a look at the Click Through Rates and the Cost Per Click on 3 different sites (Golf, Health and Wellness and Hunting) going back to 2007.  We averaged the Cost per click and the Click Through Rates across the three sites.  It is important to note that the placement of the ads and the color of the ads is different on each site but all 3 sites have ads on all the pages in very visible areas.

Year	Click Through Rate (CTR)	Clicks/ 20,000 Page Views	Cost Per Click	Earnings/20,000 Page Views
2011	1.13%	226	.30	$67.80
2010	1.58%	316	.32	$101.12
2009	1.22%	244	.28	$68.32
2008	1.93%	386	.21	$81.06
2007	3.00%	600	.18	$108.00

So what is this chart telling us?  For one thing people are clicking on far less ads today then they were in 2007.  The second big thing that jumped out to me is that cost per click is way up from .18 cents in 2007 to .30 cents in the first quarter of 2011 (Across these three sites anyway).

So back to the original question (How much money can I make with AdSense?)-  The three sites we looked at average around 3 pages per visit so it takes around 6,666 people to the site to generate 20,000 page views.  In 2011 that is about a penny per visitor.