Do I really need a citation to prove dumb people have money? Because that's basically what you are asking. Or perhaps you believe that when people click an ad on "low-quality" site the are less likely to buy than from the same ad on "high-quality" site?
Seriously, the "enlargement penis" spam works because the average person is not that selective on the ads they chose nor the websites they visit.
> Unfortunately, there is no correlation between people willing to buy stuff and high quality content.
Citation needed :)