Sheinside offers its Sheinside Women's Spaghetti Strap Contrast Mesh Flare Dress in White for $15. Coupon code "inst33" cuts it to $13.85. With free shipping, that's $7 off list and the lowest price we could find. It's available in sizes S to XL.
Note: Uncheck "Add Shipping Insurance to your order" during checkout to remove the 99 cent shipping fee. Also, it ships from mainland China and should arrive in three to four weeks.