The value of your antique mourning dress, as well as the lace, depends on age, condition, and demand. Learn as much as you can about each dress, including approximate age within 10 years and construction materials (which can help to judge age), A high-quality garment which fetches more money than a low-quality item.
For quality look for workmanship, bound seams, high-quality fabric, and the presence of silk lining.
Details and embellishments like embroidery, plisse pleats, or intricate construction can make a garment more valuable.
Holes, stains, and odors will detract from value.
Once you learn more about each garment, keep an eye on sales or auction sites for similar products. Look at Etsy, eBay, ruby lane, and Collector's Weekly. Make sure the garment you view is actually old and not a reproduction, though high-quality reproductions can be quite expensive. Look for antique clothing shows in your area. They are popular in January and February. Check out the Vintage Fashion Guild online for resources. Also, browse local shops that sell vintage or antique clothing.
Old garments should only be cleaned by a professional. Do not store in an area with excessive heat or humidity. Do not store in plastic. Wrap in unbleached muslin and refold every few months to avoid fold marks. Use lavender or mothballs to deter pests but do not let the mothballs or lavender actually touch the clothing. Store in acid-free or archival boxes.