Here is my experience how I got extra 16 GB for $18.23 (did this for two accounts, with very similar performance).
URL: https://www.dropbox.com/referrals/... (expanded)
1. Install Dropbox
2. get bonus +500MB
3. Free cloud storage: 2GB + bonus. Share and access your documents anywhere
1108 clicks, 10068 page views, 11.01% CTR, $0.02 average CPC, $18.23 cost.
Keywords: (in order of decreasing performance)
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Algeria, Argentina, Australia, Brazil, Bulgaria, Canada, China, Denmark, France, Germany, Hungary, India, Indonesia, Italy, Kazakhstan, Nigeria, Peru, Philippines, Russia, Spain, Turkey, Ukraine, United Kindom, Dubai - Emirates (city), Punjab - Pakistan (region).
Languages: Chinese (simplified); English; French; German; Spanish
Bid strategy: Manual CPC
Networks: Google Search; Search Partners; Display Network
Type: Search Network with Display Select - Standard.
Default Max CPC: $0.05