Sheinside offers its Sheinside Women's Long-Sleeve Backless Split Dress in Black for $28.67. Coupon code "SAVINGS33" cuts it to $16.90. With free shipping, that's $13 off list and the lowest total price we could find. It's available in sizes S to XL.
Note: This item ships from mainland China and should arrive in three to four weeks. Uncheck "Add Shipping Insurance to your order" during checkout to remove the 99 cent fee.