Google has introduced a new referral program which pays out 20$ per referral, Criteria for earning 20$ google says this on their site,

How you get paid
One of your visitors clicks on the graphic or link and is taken to a Google sign-up page.
That person signs up for an AdWords account or applies to become an AdSense publisher.
Advertisers qualify as completed referrals after they spend $20 with AdWords. Publishers qualify after they earn $75 in AdSense revenue.
Every month, Google calculates the number of completed referrals you have directed to Google.
Once you accrue $100 (five completed referrals) or more, you will receive a check. Google only issues checks once a month (for details, see the FAQ).

More information here: http://www.google.com/referral/

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