I haven't fully validated this idea yet, but I ran a Google ad campaign for $5 a day for a mailing list signup page that I was planning a business around. Click through rates were around 2.5%, which seems to be the baseline, i.e. a 2.5% click through rate basically means no engagement. One thing that was interesting, however, is that certain keywords had click through rates around 7%. I wanted to try re-targeting the campaign to just those keywords but I lost motivation.