When I first started out I used distilled water, I would go to the grocery store and buy it cases at a time. Since my tank evaporates on average 1 gallon a day this was a routine chore to go out to buy 5 cases a pop at ~$.50 a gallon. This equated to a $30 a month water habit between water changes and top-offs.
A quality ro/di is the only way to go, it will save you money in the long run, and as already mentioned, pay for a quality unit, it will make a world of difference in performance, and cartridge lifespan/replacement costs.