The Typhoon III 75 gpd unit is probably one of the most impressive pieces of equipment for the money that I've ever purchased. People have asked if it cost me anywhere from 500 to 1000 dollars for the thing! Considering the filter pack is 50 and the membrane is around 80, and that they throw in a free TDS meter with it....I don't know how they make much of a profit selling them so cheap.
I haven't had to change the filters on mine yet but probably will be within a few months. I don't know if the DI resin is refillable but when you buy a new sediment filter and cabon block set, a new DI cartridge comes with them, all for 50 bucks.