If there are lots of graduates, it can get expensive! We have done the $100 bill for children of our closest friends, church youth (a small group), and our DD's closest friends. I guess at some point inflation may move that upwards. In some ways, we mostly only get involved with the closer friends' graduates, but I would feel comfortable giving less, $40 or even a $25 gift card if not that close but felt I should give something. A nice congratulatory card and note and even a small gift card to me is a nice thing. That is one reason why cash has become our mainstay of such gifts, it is what it is and they can all use more, compared to picking out "stuff" and still having to decide how much to invest in that.
For "readers" we have also in the past selected a special book along with cash, but it gets harder now with e-books and such.