I have the Kent Maxxima, and it says 3:1 wasted to filtered.
I have a 135, a 20, and a 10 all running right now. I had to top off much more during the winter, and I think the total difference in my water bill (including all sewer fees, taxes, etc) went up about 30% (from $24 to $31).
That's the most I've seen mine go up, and you have to consider that we drink about 1.5 gallons a day of the filtered water ourselves too. Actually, I think we're drinking more than I'm topping off (but still much less than water changes of course)