Do you earn much from Google AdSence ?

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Do you really earn alot from adSence. How does you really earn from adsence? I have sign up for AdSence few week age but i have not setup billing information yet. I am still not very sure how the entire program works. To what i understand, we have to pay for a one time activation fee, after which we'll pay only for clicks on our ad.. I don't understand the budget:eg $15/ month..?. Does this mean that we have to pay $15 to the google to buy a place for our ad? So, we earn money when someone click on our ad. And how much do we earn when 1 person click on our ad? Please correct me if i have mention something that is not true above. Thanks in adv.