The problem is people who would pay to avoid ads are those worth targeting... I'd love if a single service could let me opt out of all tracking and ads internet wide for a fixed fee, say $50-$100 a month.
It was low enough that it wasn't really a factor when determining what to price your ad-free pages at (most people went with 1-2 cents per load) -- most people just priced at whatever average your ads were pulling in currently without worrying about the cut, since it was a fraction of a cent.