It really does depend on where you're located. Also, is she registered? If she is, her value automatically goes up a bit, and if she has great bloodlines, it goes up again. You have to think about your target audience. Is she a good children's mount? Could she be a mount for a new rider, or does she need an intermediate to experienced rider? Does she have any show wins to her name, as those can definetly be points that people are interested in. Research what type of horse sells best in your area, and if she's not the type that sells well, you're going to have to drop your target price.
I hope that helped.