I usually give $25 to kids of friends. I reserve the bigger amounts for family members such as cousins kids, neices, nephews etc. And for those family members I give $50.00.
I'm capable of giving much more so my financial situation doesn't really dictate the amount I give. I see it more of as a token to acknowledge the accomplishment rather than to help them out financially because usually their parents or familys take care of them in that way.
There was a couple of times I gave more, but the situation was that the person graduating didn't have family or were really in some type of hardship. The young person is usually thrilled to get the $25.00.