A question about paid to promote (PTP)

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I'm quite new to making money online. Not long ago, I discovered something called paid to promote on a paid to read mail (PTR) site, donkeymails. I know paid to promote means that they pay you some money to make advertisement on those approved sites. Yet, when I take a look at the price of making advertisement, I found that the money you earn from paid to promote simply cannot cover the cost of making advertisement! How can I earn money from that then? Anybody have the experience of making money from PTP?