Another hypothesis is that people interested in something would click through in hope to learn more util you ask them to pay to continue. It would mean adsense

Another hypothesis is that people interested in something would click through in hope to learn more util you ask them to pay to continue. It would mean adsense demand testing inherently doesn't work (at least for some types of products / testing setups). Someone could setups the following experiment to test it: after you build the product, split the traffic with most going to the product buy page, but with some percentage going to the original testing page that only collects email addresses. When a person gives you an email, immediately auto-reply with a link to the buy page. In this way you can compare the effectiveness of demand testing at the cost of some sales (because you make some of your potential customers jump through more hoops). But if I went a couple of months without a sale I would try it just to see if my demand testing worked at all.