I have 3 grown children and was not able to give them expensive gifts for graduation either. I gave them each the Dr. Seuss book "OH The Places You'll Go". I wrote a note in the front telling them how proud I was of them and how much I loved them. I made a wish for them in the note that they would go "All of the places they wanted to go" and do all of the things they wanted to do. Each of them loved it so much it is now the graduation gift I give all of the special young people in my life.
Just a foot note...Each of my children took that book to the hospital at the birth of each of their children and read it to them right after they were born then wrote a note to the child under the note I wrote to them.