Generally, I don't give money unless I don't know someone. Even when I do, I usually give a gift plus a little cash. It may be that most of my friends and family are in more rural areas (not same cost of living as Chicago area), but I tend to spend $30-75 for high school graduates depending how well I know them or relationship to them. More than that for a college grad and no more than $25 for 8th grader. I hope I'm not being cheap, but I have kind of thought from others friends/family that was kind of the norm for their areas.
Typically, for an 8th grader, I would stick to a small gift of some type. The closest I'd get to cash is an itunes gift card or movie gift card. For the highschooler, I might do a combination of gift cards (itunes, gasoline, Starbucks Target) or a gift. My best friend's daughter adored a small Coach clutch I bought at the Coach outlet for about $40 and talked nonstop about it. When my cousin's daughter graduated last year, my mom and I together spent about $100.