My daughter's graduation from high school this year. In my mind HS graduation is expected - it's not as special as college graduation unless there have been extenuating circumstances like a serious illness or death in the family, disability, etc.
I think cash is the est option since kids going to college have so many expenses and every little bit helps. But I think $25 is more than enough for friends - invited to a back yard party. If it's a catered event at a restuarant or soemthing like that - then it's an entirely different situation. For family I think $50 is fine. And I live in NY where everything is more elaborate and pricey. I think some of my NY friends and nieghbors would think that a higher amount would be more appropriate - but I don't. College is another story however since that requires a sacrifie of time, money, effort, perserverance, etc.