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.
You have misread his comment. Look at the last bit:
> 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.
I think you have misread my comment. A regular toothbrush + toothpaste costs pennies a day, add floss and mouthwash and it would still not reach $20/month. If I can take care of my teeth by using all of those costing so little, what on Earth would I gain by paying more?
If he wouldn't pay $20 a month to prevent tooth decay, he clearly is the type who knows the price of everything and the value of nothing.