Amazon offers its Prime members the Sawyer Mini Water Filtration System in Blue, model no. SP128, for $17.92 with free shipping. (Sports Woot offers it for pennies more. Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $5.58.) Aside from the mention below, that's the lowest total price we could find by $2. It filters up to 100,000 gallons by removing 99.99999% of all bacteria and 99.9999% of all protozoa. It attaches to an included drinking pouch, standard disposable water bottles, or hydration packs.

A close price: CoalburnOutdoor via eBay offers this filtration system for $19.40 with free shipping.