Arteesol via Amazon offers its Arteesol 5.5" Smartphone Arm Band in Black for $10.88. That drops to $3.92 during checkout. With free shipping for Prime members, that's $22 off and the lowest price we could find. It's waterproof and features a reflective strip, key, card, and headphones holder, and adjustable Velcro.

Update: You may need to apply coupon code "3PUZKQOO" to see the discount.