What are the images worth to you? Is this something you do for fun and not for profit? Will the loss of the images mean you lose high quality material for your portfolio/website/brag book? How much is she willing to pay?
By selling off the copyright, the images become all hers. She can then turn around and sell them, lease them, license them to whoever she wants for whatever reason she wants. She OWNS those images and the rights to them. Will this be a loss for you?
My take is that if you are like me, and do this for fun (if you were a working pro, you wouldn't really be asking this question), figure cost of film and processing (I know, an assumption!) and your time, and multiply it by 100. Then negotiate from there.
For one of my normal shoots that would be 5 rolls of film at $25/roll (36 exp) + 4 hours at $50/hour x 100 = $32,500. If ownership of the images is worth that much too her, she'll negotiate. Perhaps she wants just one image. Do the math -- about $180 per image. Then again, if that one image is that good, you could stand to make substantially more than that licensing it for stock (provided you have a release from the model).
Frankly, unless the payoff was substantial, I'd keep the copyrights.