Which toothbrush is so much better than a regular one that it would be the difference between tooth decay or not? It's mostly up to how often you brush and floss, not how many LED's your toothbrush has.
> What kind of crazy value would you expect something to have in order to be worth $20/mo?
The implication of that is that of tootnhcare cost more than $20 a month he wouldn't do it. Which is crazy, the expected return from brushing your teeth is well above $20 a month.