I think that nicole has a good point.
I would charge her at least $20.00/purse plus the cost of the yarn - you had to learn so you need to charge her for your expertise, too!
Though if she's ordering alot, perhaps give her a discount.
realising I'm absolutely no help whatsover! I just think us artists tend to undercharge for our talents
EDIT: I was posting at the same time as Ingrid - Ingrid has a good point, too