While Paying Upfront in Online Earning sites we must be extra careful

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I come across many online websites which shows great earning online opportunity by paying some amount upfront while registration itself. I doubt, why such websites will be charging such small amounts, when earning such huge amounts is possible for sure. I have paid such subscriptions 2 times, of course not very big amounts but unable to earn any thing. Now I came to know that we should try only those sites which offers free registration and earning opportunity and only after earning some amount and we are confident of making money on working with that site, we can go for any plans that require small investment and which boost up our earnings. Do you agree? or will there be any real genuine sites where earning is possible only if we are to pay some upfront amount, if so, how can we believe that site is really genuine?