Google, of course, keeps a great deal of secrecy regarding how much AdWords advertisers pay per each click directed to their site and the same applies for how much AdSense banner holders make from their websites.
While there's nothing official, rumors circulate around the Internet concerning the amount of cash a website can earn by using AdSense. And many people (illegally) disclose how much they have been making with AdSense. There are stories of people raising over one thousand dollars per month using AdSense.
There are also stories of people exceeding $100,000.00 per month but it's a bit hard to believe such stories. The truth to the matter is that if you have a small website and you just want it to support itself, and don't wish to reach your pocket for its maintenance costs you can probably do this with AdSense.
AdSense is also very good for people who host a lot of pages. Even if the said pages don't generate a lot of traffic individually, every click counts and you can earn up with a lot of money by doing this. And that just goes to prove that sometimes quantity matters nearly as much as quality.
There's no telling how much money you're going to make by using Google's AdSense but you can sort of tell for yourself, before actually starting, by taking a few things into consideration.
First, is the amount of visits you get every day. While there's no way to estimate precisely on this, you can generally make a safe assumption that if you have a lot of clicks per day you'll be making good money.
Also, this depends on what exactly your site is about. If your site is about anything popular (music, sex, whatever) you're bound to get a lot of banner clicks. These have a coefficient associated with them, called the CTR (click through ratio).
Basically, what it translates to is that if a large proportion of your site's visitors click the ads you'll be making more money. And the best way to do this is to have some popular content in your site, ensuring the links direct users towards popular items as well.
Then of course, there's the position and number of ads on your website. While you don't want to overdo it, having many links will undoubtedly generate more income for you as a webmaster. Do not however believe, that if you just add a lot of ads in an important portion of your site, visitors could always just skip them (and be assured that many do just that).
There's something between an art and a science to positioning your ads. People generally look in certain places and never look in others, and knowing this a website author and/or webmaster can do a great deal of things to increase his earnings with AdSense.
All in all, the amount of money you make with AdSense depends on many factors. But if you have a site with interesting contents and/or many pages, and if you see a constantly large amount of traffic every day, you can bet you'll be making a lot of money with AdSense.