Amazon offers Prime members the GoPro Blackout Housing for Hero 3+ for $26.99 with free shipping. (Non-members can spend $35 to bag free shipping; otherwise, it adds $5.23.) That's the lowest total price we could find by $3. It features a matt black finish to reduce light reflection and is waterproof to 131 feet.
Update: Note that this item is temporarily out of stock, but can still be ordered at this price for delivery when available.