SheIn offers its SheIn Women's Informal Long Sleeve Split Maxi Dress in Black for $19.99 with free shipping. (It's available in Red for a buck more.) That's $16 off list and the lowest total price we could find. It's available in sizes XS to L.
Note: Uncheck "Add Shipping Insurance to your order" during checkout to remove the 99-cent fee. This item ships from mainland China and should arrive in three to four weeks.