The price per gallon varies greatly from dispenser to dispenser - but the best price is from units that dispense into your own container (as with many things, the container/packaging is often the most costly component). My local grocery store (chain) had it for $.29 a gallon at one time, but it kept creeping up and when it went over $.65 a gallon, I went with a home RO system and haven't regretted it. I can collect 9 gallons a day and I simply keep it in 5 gallon water jugs until I am ready to use it. Then on the day of brewing, I treat according to Brun Water and my beers have been the better for it.
YMMV - good luck and Cheers to your move to better water (and hopefully better beer!)