Quilt Appraisers base the value on quality of fabric, design, colors, pattern popularity, and the quality of the quilting itself. She looked for damage, fraying, what kind of binding and backing was used, etc. It took most of an hour for her to thoroughly evaluate it, and she took several photos.
And she told me that Stack N Whack quilts are really not of lasting value - they are a fad. I guess I won't take a class on that one!
Patterns that have been around forever are the best ones to make if you want a valuable quilt.