In this case I don't think there is an "appropriate." What do you want to do?
Sounds like you would like to mark the occasion but since money is tight you aren't comfortable with something pricey (which I think is unnecessary anyway). So I would suggest coming up with how much you want to spend and go from there. If the dollar amount is zero then write a really nice card or poem. If the dollar amount is very small, a token or appropriate svarowski crystal of some kind, or a gift certificate at a nice restaurant will do.
I am also wondering what the degrees are in. Maybe you can come up with cute gifts in regard to that. I recently gave a graduate planning on beginning medical school a stethoscope. A lab coat would have been cute too. So, what's their thing?