Considering that the cost of raising a child in this day and age runs somewhere between $200,000 & $400,000 and children have 20 baby teeth. The cost of that first tooth should be somewhere around $10,000 - $20,000!
Of course, the tooth fairy isn't made of money so it might take them about 18 years to come up with this sort of funding. In the meantime, with timely monthly payments and some crude investing, inflation shouldn't affect the long terms costs too much.
As for a down payment due at the time the tooth is removed. I'd guess somewhere in the $1 range depending of course on the current financial situation. Just don't let them buy that chocolate bar with the money. Just remind them, "Ten years from now I can either buy you new teeth or a new car!" At six, they'll definately choose the chocolate over the car.
Whatever the tooth fairy decides to leave, make sure Mom gives them much more. :)