A very subjective topic. Adsense keywords all vary tremendously. Depending on niche, keyword, or phrase they can vary from a few pennies to $10 or $20 or more. Generally speaking, adsense keywords are valued based on their popularity. The more competition for the keyword, the more it will cost advertisers to ‘place’ the adword ad.
The flip side of that adword ad. Is your adsense account. You will earn around half of the advertisers cost for that click. So it would make sense for a blogger or webmaster to target the valuable and profitable keywords.
The next step in making good $$$ with adsense? CTR or click through ratio. This is based on how many ads are shown in proportion to how many clicks you log.
On the forums I have been frequenting CTR of %2.5 to %5 or more are considered excellent. Good writing, popular topics, correct adsense placement on page and proper SEO efforts can get you there.
Stay tuned for more on this Adsense experiment.