There are a host of issues when considering RO, DI, RO/DI water systems.
For RO, as 1077 mentioned, even the best require at least 4g of source water for each gallon of RO water. A fair amount of waste.
The system must also have sufficient pressure which may not be an issue for most municipal water supplies, but often require an additional pump for well water systems. Then there is pre-filter and membrane replacement costs.
RO, DI, RO/DI water is too pure for aquarium fish so mineral additives and pH adjusters are required and these are not cheap.
Finally, the process is not on demand, producing only a gallon or two per hour, so storage tank(s) are required. As you can see, the cost of a good RO water system can get a little steep. Purchasing RO, DI, RO/DI at <= $.50/gal might be an option? But then there's the drudgery of hauling water!
So we need to evaluate the inital cost of the RO system, maintenance as well as the on-going cost of mineral and pH conditioners. Combined, these would likely exceed the current cost of Prime.
Also, when comparing the cost of Prime to other conditioners, I think we have to look more closely than unit cost because Prime is much more concentrated than most other conditioners. We need to explore the cost per gallon of conditioned water. Often Prime will condition twice as much (or more) tap water than competitive products.
I think that unless there's some other reason to suspect your water quality, you are best served to continue using Prime or another effective conditioner.
To reduce costs, perhaps you might examine your current filtration/purification systems as well as tank maintenance such that you could somewhat reduce the volume and/or frequency of partial water changes?