All I know is I just got back from the LFS and overheard the owner quote $1.50 per gallon for his saltwater. I did not try to do the math but it sure seemed high to me and judging by the video it turns out to indeed be quite high.
If you are actually buying saltwater and not salt it will be much higher.. In some places like where I live water is not cheap so you add the cost of water. If they use a RO unit to purify it then it wastes allot of water too.
Now you have to add in the cost of filters and the actual cost of the unit. You have labor and storage. Well and obviously the cost of the salt...