But why would anyone opt-in to that? You could do what some site are doing put up a "Either pay us or opt-in to cookies and tracking". The problem is I don't think people fully understand how much tracking we're talking about.

Right now a Danish radio station is running a number of news stories about being able to track people who work for military intelligence or as police officers and prison guards. They do this using a free sample a data broker provided. Everyone act surprised when the journalists are able to show up at the home address of military personal or prison guards who have their home address protected/secret.

I don't think there's a safe way to opt-in to selling your data, because most people cannot comprehend how much data, what type or the ramifications.