A little secret? It all gets thrown away. Some sooner than others, but in the end it's usually trash. I wasn't thrilled with the idea of just throwing away money (sometimes the sets can be pricey, and if you have more than 18 or so in a class, then you need two boxes!), so I started working with the kids to make their own. We use what we already have, so that saves the expense of buying cards or supplies, and I let the kids go to town.
I think it helps them to put thought and effort into a gift, they didn't just sign their name. It also entertains them ;)
I've decided to do this for other occasions as well. Why buy a card when the kids are more than happy to make one? It isn't a big savings over the course of a year, but I think that message it sends (to my kids and our recipients) is worthwhile.