I did $5 for the first one ever, however, weeks later she had a meltdown and demanded that the tooth fairy return the tooth. She even tried to put the $5 back under her pillow🤣.
It depends on how many small bills the tooth fairy happens to have on him at the relevant night.
I'm only a little ashamed in admitting that, one day when only $20s were in the wallet, I may have snuck into the LO's room, grabbed a couple of singles from her piggy bank, and then had the tooth fairy give her those (supplementing later, of course).
We went down this route, and it's one of my big regrets. Was AU$2 a tooth and a wish, which started as an easy and cheap soft toy (think a couple of bucks max). But it's turned into $2/tooth cumulative, ie. Tooth 10 gets $20, tooth 11 gets $22 etc, and a wish.... Cue late night runs to Kmart and lots of grumbling about why we are doing this.