I don't know why you'd buy water anyway. How many gallons are you going through in a month? Let's just say you're using 10 gallons. So generally the cost is $1 per gallon or $10 plus tax....just for the water. Now you have to figure in the cost of gas to go get the water. On top of that...you have all the labor of carrying it from the store to your car....then to your house.....then to the tank.
For around $100 (or maybe even less) you can get a high quality RO/DI unit. The short payback is 10 months but there's way more to it than that.
To me, an RO/DI unit is as important as any other piece of equipment you can purchase when setting up a tank. Filter changes are around $40 a year...maybe longer depending on how much water you use in a year.