Am I eligible to ask users to click adds to gain some in-app benefits?

[Sergey Bugrov]

Is it "legal"?

[galeksic]

It is not legal, and it can result in your account termination.

Invalid click activity consists of any clicks or impressions that may artificially inflate an advertiser's costs or a publisher's earnings, and for which we decide not to charge the advertiser. This includes, but is not limited to, clicks or impressions generated by a publisher clicking on his own ads, a publisher encouraging clicks on his ads, automated clicking tools or traffic sources, robots, or other deceptive software.

https://support.google.com/admob/v2/answer/3342054?hl=en&ref_topic=3342053