On a typical day the amount that an average visitor "pays" $0.002 to view a bit of content by a certain percentage clicking on an ad. Through advertising, that bit of content has a value of $0.002.
True, but what has it to do with ad blocking? The typical AdBlock user never clicks on ads, that's a zero percentage, yielding an average $0.000 a day for viewing said bit of content. Unfair, you might say, but then, what do you want to do about it? Technically prevent "never clickers" from accessing your content? Impossible. Introduce a law that requires at least one click'n'purchase a month? Ridiculous (well, not necessarily, I can envision an RIAA-supported bill that requires every US citizen to buy at least one music CD a month).
Those users who do find your ads interesting, though, will not use AdBlock, or at least not configure it to block those ads that they DO find interesting.
Yes, I admit that's a shameless simplification regarding current ad blockers, which lack sensitivity. This problem could only be solved through cooperation between ad blocker developers and advertisers.
Of course, it seems like ever-more fierce confrontation is likelier than cooperation, and that's, unfortunately, human nature. Just consider the ongoing RIAA/MPAA madness. So my prediction is that, as ad blockers are used more often, advertisers will first increasingly try technical measures to circumvent them, i.e. deliberately reducing sensitivity, which will result in ad blocker developers finding ways to stop circumvention, and users blocking more ads than they actually would like to, meaning losses for advertisers, and thus reduced advertising revenues for content providers.
Should the advertisers start to see themselves in a losing battle, they'll increasingly use legal weaponry (and lobby legislators for more such weapons) such as copyright to target ad blocker developers and eventually also users (just like the RIAA). This will still be a losing battle, but it'll extort a heavy toll on both sides (just like the RIAA/MPAA war on P2P), with no end in sight.
All this insanity will cost a lot of money, further diminuishing ad revenues for content providers, and many will find advertisements no longer viable. By the way, the IFPI (International Federation of the Phonographic Industry) has recently tried to reduce the copyright holders' share in German CD prices, stating that they need these 2 or 3% of the total price to "combat Internet piracy". So, like every war, this will be a vicious circle, the longer it wages, the higher the stakes.
Long story short: It would be nice if we could all just get along. Unfortunately, it's not in our genetics. Let the games begin!