http://www.amazon.com/dp/B00FBGCLSC/ref=twister_B00FHRADQ2

Just bought the four pack for $65. Camelcamelcamel price tracker lets you know when the price drops. One for you, one for your mate, one for the car, one for the home emergency kit. Can be clipped inline to a bladder, attached to a soda bottle, or straight from the stream. All for under a pound. You're the hero when someone forgets the water filter or I'd you're in a sketchy water situation. back it up with some aqua Mira tablets just in case you happen to freeze/ smash four filters at once.