PTC Site Investment Requirement (not upgrade requirement)

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I have been considering, perhaps foolishly, investing at bux sites that upon attempting to cash out, point out the small detail that you need to give them money to get money. A typical requirement would be, you have made $1 by clicking, they then tell you are eligable to receive say %200 of what you invest in the site. So say I give them $.50, then I a) Get $.50 in purchase balance, b) paid my $1 for clicking. So in the end, I make $.50 and purchase $.50. First, do I understand the transaction correctly? Second, I have this sneaking suspicion that when I transfer my $.50 to them, they very well may give me $.50 in purchase balance for advertising, but just won't pay me my $1 for clicking. Anyone out there have first hand experience of investigating and having this happen or not? Thanks!