You could try asking at your local quilt shops if they know someone who does quilt appraisal. Sometimes at our quilt shows, there will be someone who will do appraisals by appointment. They charge a fee to do the appraisal.
As to the quilt you are being asked to make, especially if you are hand quilting it, $800.00 is not nearly enough $ for you to charge. That "might" cover the actual quilting, but not for the materials. If it took you 80 hours to hand quilt the king size quilt, you would only be getting $10.00 an hour for your time for the quilting and that wouldn't even include the piecing time...which would be many more hours. I agree with others...have her purchase all the fabric for top, back and the batting and thread, and then charge a fee for doing the piecing and the quilting. (3x cost of materials as a starting point, but with handquilting, it may be even more)