Do you think PTC website owners earn good income?

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I'm planning to buy a ready-made PTC site, but I'm not sure if it's worth it or not. Do you think that I may earn nicely from the site or should I forget the whole thing? I downloaded the book that gives all the information as a beginner in the PTC "industry" and the book is really promising good money in that, but still I have my doubts. So what do you think, any advise will be very very appreciated :)