P.W.
I think the norm is 50 cents, I've seen it be $1. If you do 50 cents, make the cups smaller.
Most people buy to give the cute little kids money, so I think they'd be willing to pay $1. I remember one time my oldest, who was then 8, caught some bugs with his friend and they were selling bugs for $1. It was so cute! My neighbor bought them all.