GOOGLE ADMOB is a program made available by Google to APP publishers. It give publishers the opportunity to earn using their earning app.
GOOGLE ADMOB pays publishers since it was started and still paying up till now. Google Admob pay per click or ads(advertisements) views but don't pay for self clickers.
GOOGLE PAYS YOU ON EVERY 21ST OF EVERY MONTHS.
What does it cost to EARN with Admob? Just Your App and internet Connection.
For Tricks and more about Admob click here