Monoprice.com offers its Monoprice Weatherproof Hard Case, model no. 12180, for $15.19 with free shipping. That's the lowest total price we could find by $18. It measures 7" x 6" x 2" and features customizable foam, IP67-level dust and water protection, a shockproof design, pressure relief valve, customizable name plate, and lock holes. It is waterproof up to one meter for 30 minutes.

Of note, it's also available in a larger 8" x 7" x 4" size, model no. 12678, for $15.99 with free shipping. That's the best price we could find by $17.