Nobody can take advantage of you, unless you allow them to. When you send her the business cards, enclose an invoice for them.

In my line of work, I make business cards all the time, and my prices are:

100 Full Color, printed and cut: $22.50
250 full color, printed and cut: $37.50
500 full color, printed and cut: $72.50

The reason she makes you do them, is because she gets them for free, and she can't go anywhere else cheaper. Nothing in this life is free. AND, she can get what she pays for. If she doesn't pay you, I would print them out in black and white, and on the lowest possible quality.

To give you a little perspective: my mom is a seamstress, and she has a neighbor that likes to ask for "favors". My mom worked on a 3 hour job, and asked for $15. The neighbor got so offended that she has refused to talk to my Mom since. The thing is, nowhere else could she have a custom job done like that for $15, and she was only taking advantage of my Mom. Nobody needs a "friend" like that!

I hope this helps you a little bit, but I definitely would not do anymore printing "for free" for her. If she pays you, or returns in like kind, then that is a different story.