I had never even heard of this product before. Honestly it isn't making any promises other salt mixes don't make, other than the bacterial content, and it's my understanding that products like that have no actual extra benefit, like "live sand" in a bag.
At that price it's kind of ridiculous. Regular salt mixes have been working in peoples reef tanks for quite some time, and it's cheaper.
I would try to find a few serious reefers who have actually used this product.
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).
Consider when you're shopping for an R/O unit how many gallons per day it makes. 100 gpd runs about 5 gallons every hour and a half, give or take. So... it's going to take half a day for a 10 gpd unit to make 5 gallons.
Unfortunately, the GPD is also a function of your house pressure. We have the minimum here (about 60 psi) so my 110 GPD only puts out about 1 gallon an hour. None of the 110 units will produce the promised output for normal house pressure but ours is particularly low.
I'll tell you, I spent a LOT of money of F'ing water. I bought spring water from Publix at $13 per 5 gallons with the deposit, then the salt was another $60 to fill a 92 gallon tank. I wish I'd have just bought a damn RO machine. Part of this was due to me being a little impatient/worried about letting the live rock sit without the pump on too long, so I just shelled out the dough.
My local fish stores all sell pre-made water for about $1.25 a gallon. That's a lot easier to take, and I kept three of the jugs from my initial investment for transport and they are wonderful.