I have no answere for you but its a real cool thing to have to think about.
In my day it was a coin and a quarter was considered real good! Most often it was a dime that could be slipped into a little slit in the corner of the envelope and the tooth removed from the same little slit right along the fold. That way the kid will think it must be magic if he seals it in an envelope himeslef before going to sleep. next day , tooth gone , money there in the same unopened envelope.