What you describe is exactly how my product works (FairBlocker). We split up a user's chosen subscription among sites where they block ads... I imagine it is what Contributor is doing, too. The way it is working out for us is that a ~$6.5 / month subscription is equivalent to a ~$12 net CPM, though it varies depending on how heavy a browser the given user is. So, higher than what many small sites normally make, but also lower than many video ads or top-tier sites with their own sales teams.