ROI help

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I have come across several PTC sites that have a "guaranteed" ROI percent listed. Is this the maximum amount you can earn? Is this maximum per cashout? How does it work with free members who don't buy referrals or ads? One site I saw a 125% ROI but a 25% FEE to cash out for a free member. To me this doesn't make ANY sense. Is this right? If you buy adds worth 10.00, you can only cash out 12.50 but get charged a 25% fee which means you only get paid 9.38 (Then paypal takes their fee if your account is business). OR does that mean you can cash out multiple 12.50's on the ONE 10.00 investment?