What is the AdSense revenue share?

For displaying ads with AdSense for Content, publishers receive 68% of the amount Google collects from advertisers. For AdSense for search, publishers receive 51% of the amount collected from advertisers. These percentages are consistent, regardless of a publisher's geographic location, and are not in any way averaged between publishers. We don't disclose the revenue share for other AdSense products; the revenue share varies for other products due to different costs of developing and supporting these products.
You can view revenue share details in your account:

1. On the Home tab, visit the Account settings page.
2. In the "Account information" section, you'll see each revenue share displayed next to Active Products.

We believe our revenue share is extremely competitive. However, revenue shares alone can be misleading, so we encourage you to focus on the total revenue generated for your site. For example, if the Google ads on your site generate $100, with our 68% revenue share you'd receive $68 through AdSense. Another ad network might offer an 80% revenue share, but only collect $50 from advertisers, so you'd receive $40.
With the vast number of advertisers competing to appear on AdSense sites, our system ensures that you're earning the most possible for every ad impression you receive.

Source:  http://support.google.com/adsense/bin/answer.py?hl=en&answer=180195&topic=1391547&ctx=topic