K.R.
last time I saw this go by a lot of people used those gold-colored US dollar coins, which I thought was super cool but $1 feels like too much to me and I'd already established a different pattern before I read that
we give fifty cents--I always use 5 dimes because it feels more fairy-like to me than two quarters, but their dad I think uses quarters at his place
sometimes the tooth fairy doesn't make it on the first night (tired parents!), so they've also received a few handwriting-altered apology notes with their 50c the next day. I sometimes wrap the 5 dimes in a stack with a bit of shimmery ribbon.