That's a tough one. I knit for friends all the time at no charge if they provide the yarn. I think the most I've ever done was when someone asked me to get the yarn I ballparked it and told them a price and we agreed I would keep the balance if I was able to get the yarn cheaper.
I guess it depends on how friendly you guys are...maybe charge her $5 for the small and $10 for each of the larger ones. What were you initially thinking?