Ha ha, 300 gpd! I only just looked at the specs. Man I really didn't know what I was doing when I bought it. Oh well!
Yeah you're right, that is expensive for basically a 300gpd unit.
Well maybe I'll just keep it, and rig up those 2 booster pumps to my 4040 RO instead. (I have pretty rubbish mains pressure of 32psi) I have ordered a new power supply for them (£20), I guess the pre-filters are fine too. It must just be that little RO membrane doesn't produce much!