AdSense launched a section for new publishers - Newbie Center - where you can find a lot of basic things about the program, but also read some numbers. Google uses "might" whenever there's a number involved probably because they don't want to influence you too much.
"While there's no magic formula to determine how much revenue you'll receive based on a certain amount of traffic, it helps to be realistic about your earning potential. For example, for every thousand page impressions you receive, you might earn anywhere from $0.05 to $5.00." A 2006 article from Business 2.0 Magazine partially agrees to this: "CPM rates on Google AdSense and competing automated systems are estimated at anywhere from 50 cents to a few bucks."
Google also talks about the magical high paying ads: "while it's nice to receive higher paying ads, keep in mind that higher paying ads may be aimed at a smaller target audience and therefore generate less interest and ultimately fewer clicks." Google says that "for a larger site, 1% might be considered a decent clickthrough rate".
Google AdSense's TOS doesn't allow you to reveal the clickthrough rates for your sites and, by default, Google displays stats at the page level. So instead of seeing the average CTR and CPM for an ad unit, Google will show the average numbers for a web page. To see the real numbers, you need to go to advanced reports and select "show data by ad unit". Note that Google doesn't break down the data by domain or subdomain, so you need to create URL channels for each of your site. You can create up to 200 URL channels and they start tracking data only after you add them.
Google AdSense makes it difficult to find key information about ads and it doesn't separate the performance by ad block type, domain, referral etc. At some point, a post from Google Analytics blog suggested a possible integration with AdSense, but the current level of openness is not encouraging.