Woot offers the LifeStraw Personal Water Filter for $14.99 plus $5 for shipping. Aside from the mention below, that's the lowest total price we could find by $3, although we saw it for $5 less in June. This award-winning, BPA-free device enables you to safely drink contaminated water. It purports to remove almost 100% of waterborne bacteria and protozoan parasites and will filter up to 1,000 liters of water.
A close price: Water Check via eBay sells this filter for $20.95 with free shipping.