Our standard gift for a non-relative is $25. I coach a high school team - so we've been to many HS grad parties. It is what makes sense for our budget. I think this year - we are going to a total of 6 parties, between team members, and acquaintences.
Now - this year, one of my team members graduated. She has been on my team for 3 years. I went a little higher - I still gave my standard $25, however also got her a Vera Bradley mini-hipster, that I got on a huge on-line sale last fall where there was free shipping. (This person loves VB purses, but I thought in college- she might want something a little smaller at times.) This young lady, IMO went above and beyond as well. Her parents were very supportive of the team in many ways as well. So - special circumstances IMO led to a "bigger" gift.