I believe that Walmart sell them if that suits you better. If your tap water is about 50tds then you should get 150L approx per filter. Also it won't matter if the tds/GH/Kh of the tap water fluctuates as the filter will remove what is there, it will affect the life of the filter though proportionately, so probably not a great deal. You will probably get a tds meter with it!
The one thing I have noticed with it is it goes almost to the end of its life at 000 and then 001 for a short while and then quickly up to the 006 limit whereby they say you should change it. It isn't a problem as checking the tds is quick and easy, it is obviously just the way the filter works, ie once it has reached its saturation point it loses the ability to work efficiently.
Based on a jug for $26 and 150L that makes 17c per litre (it will come with a filter) unless my math is out, and 7c per litre after the first filter?? Probably a no brainer..........
Simon
Simon