OK, I was actually in Google Ads (the first time) 2008-2010, and saw the results of zillions (technical term) of experiments, where pCTR (probability of a click-through) was one of the variables measured against the control.

You're right -- it's nothing like 30%, except for maybe some extremes, e.g. "mesothelioma" where the user really wanted that information.

A few percent.