Amazon offers Prime members the LifeProof Fre Waterproof Case for Samsung Galaxy S5 in Teal, model no. 2401, for $9.99 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $6.22.) That's the lowest total price we could find by $5, although most retailers charge $32 or more. It's purportedly fully submersible up to 6.6 feet for one hour.
Note, this item ships within two to four weeks.