From the replacement cartridge, that is about half the price of buying the 5 gallon RO water. You can get 20 gallons for 20 bucks. So add a buck to that for 40 gallons to pay for the unit, and that's not bad,
if it is easy, and works like it is supposed to, and you do get RO water out of the unit and cartridge. You would have to know the PH, TDS, check for metals, chlorine, etc, against 4 stage filtered RO water.