A lot depends on if you have a LFS that will sell water. Here, IF you can find one that sells it, it runs closer to $1/gallon but often you can bring in your own container and get it cheaper. The chain stores charge more but what they sell, they don't mix there and have to buy and pay for containers and shipping (do ask if store mixed is available though, even some chains may sell it that way if you talk to someone).
If I understand the initial question correctly, I think this may be the first premixed Cephaholic has found and may not know that there are alternatives. Try calling a local fish stores to see what is available (it won't be out on the shelf). Check to see if they sell it more cheaply in quantity, a salt bucket makes a good (but heavy) porting and storage container if you can find someone that has empties. You might also try your LFS for availability/price of unboxed RO water (again, you will need to find/buy container(s) once for your best pricing).