I do like the Sawyer filters a lot. Their SP 135 filter, which provides virus filtration to 99.9997% and biological filtration to 99.9999%, is a great piece of gear. It should be noted that this filter can be used inline with quite a few different water carry systems. It can also be used on the fly with no need of pumping action only by drawing the water through the filter with your mouth.

You can buy the Sawyer Purifier kit at Wally World Online for $59.00. It is the same filter you get with the gravity feed set-up. So, if you already have the other gear, or you want to put together a DIY gravity feed filter with a couple of food grade 5 gallon buckets, you can do it more cheaply.

Standard Disclaimer: I don't work for Sawyer or any retailer that sells Sawyer products.