SheIn offers its SheIn Women's Spaghetti Strap Sheath Dress in Black for $9.99 with free shipping. (Uncheck "Add Shipping Insurance to your order" during checkout to remove the 99 cent shipping fee.) That's $5 off list and the lowest total price we've seen for such a dress in black. It's available in sizes S to L.
Note: This item ships from mainland China and should arrive in three to four weeks.