There is thing on Internet called GOOGLE. It will provide you with all the information you need about making money through adsense. However, before adsense, there are tons of thins that you need to know. Google is the source of all knowledge. Start using it
Yeah its completely depend on niche and keywords. And also depends on geo traffic. If your site running based on US, then there is a great chance to earn more from adsense. Google pays more CPC in US traffic.
Highest paying keywords like: mesothelioma law firm, online insurance, etc Research yourself at: https://adwords.google.com/o/Targeting/Explorer
I think the highest click I ever got was $5. I get the occasional $2 click but most of mine are < .50 I have yet to crack the adsense nut.. but then again, I have yet to take a real hard swing at it..