a house by house opt-in system would be tremendously more expensive
Nope -- it's called fluoridated toothpaste or fluoridated mouthwash.
In general, the cost-benefit analysis for centralized interventions is difficult. Even besides the issues of properly estimating them, the costs and the benefits rarely align neatly -- there are winners and there are losers. The problems are exacerbated by the fact that the powerful usually make sure they end up among the winners and the powerless often enough find themselves among the losers. I tend to be wary of "it benefits everyone!" handwaving.
Nope -- it's called fluoridated toothpaste or fluoridated mouthwash.
Which is only ten times more expensive. It's also more effective than just drinking fluoridated water, as you might expect, so the comparison isn't that clean.
A post from Gregory Cochran's and Henry Harpending's excellent blog West Hunter.
The commenter Ron Pavellas adds:
The Wasserman Test.