1) you don't get cost per click, you get earnings per click(epc) 2) nobody knows the true answer to your question, except for google
The people who created the ads that are being displayed on your site are all bidding different amounts per click to have their ad displayed. Google uses their bid to determine their ad placement and frequency. Their bid also determines in part how much Google pays you when someone clicks on the add.
This is how AdSense works. If you want more dependable revenues, you should try something else, like the audio ad network I use on my own sites.
Because keyword, advertisers, competition, etc. are not constant. Sign up for Google AdWords or read something about it and you will understand everything.
It depends on geographical area also. A click coming from US might cost $1.00 and the same click coming from Bangladesh might get 10 cents.
I get a few clix only so i can monitor 'em closely Got first clik today for $1.48 - then another for 14 cents - then there were 4 clix wid total earnings pf $2.96