I recently dealt with the same experience that you are going through. It is difficult because you don't want to overdo it. I finally decided to give my daughter two dollars--a dollar coin and a silver dollar--I printed out a cute little "tooth fairy" note telling her that she was receiving this because she lost her first tooth and for keeping them sparkling clean! It was less than a week later that she lost her second tooth. I left her a dollar for that one along with a similar note.
I know that at times I can be really overwhelmed with my 3 children and sometimes forget things. I really didn't want to start something that I could not continue. It can be hard to know what dollar amount. I have seen parents give $5, $10 and even $25 a tooth. I think that the most important thing is to be consistant with whatever you chose. At times I wish I would have started at .50! My children really need to learn the value of money--it concerns me when I hear, "Man, a dollar isn't anything!" from some kids.
Best of Luck!