What is the current state-of-the-art for portable, personal water purification methods and devices, like the sort you could use to treat a bottle's worth of wat

What is the current state-of-the-art for portable, personal water purification methods and devices, like the sort you could use to treat a bottle's worth of water? Devices like the LifeStraw [1], the LifeSaver bottle [2], the WaterIsLife straw [3], and the SteriPEN [4], are nice, but they're pricey. Any ways of reducing the cost? [1] http://lifestraw.com/ http://lifestraw.com/ [2] http://www.iconlifesaver.eu/ http://www.iconlifesaver.eu/ [3] http://waterislife.com/clean-water/the-straw http://waterislife.com/clean-water/the-straw [4] https://www.steripen.com/ https://www.steripen.com/