Shoppers compare salt by the bag, but a bag is a poor unit of comparison. Two households can buy the same bag and spend very different amounts over a year, because the salt disappears at a rate set by the hardness of the water coming in, how much water the household uses, and how often the unit regenerates. Price the salt against the water it actually softens.
The four inputs that drive the salt bill
Hardness comes first. Get a water test that reports hardness in grains per gallon, and write that number down. Do not rely on a guess or on a neighbor's figure, because the salt needed scales with it. Second, measure household water use. Read the water meter at the same time on two or three mornings, note how many gallons passed between readings, and use that as your working average. Third, look at the regeneration setting. A unit set to regenerate more often than the water requires uses more salt. A unit set too rarely lets hard water through, which brings the scale and soap problems back. Ask the installer or technician to show you the number on the screen and tell you why it was chosen. Fourth, consider the salt form. Pellets, crystals, and block salt each dissolve at a different pace and leave a different amount of residue, so check the bag weight and how much residue appears in the brine tank over a few months.
Do the math once and keep it
Count the bags you buy in a typical month and multiply by the weight on each bag to get pounds of salt per month. Divide your monthly salt spend by the gallons you softened in that month, using your meter average, and you have a cost per softened gallon. Redo the calculation after any change in regeneration setting, salt type, or household size. Two bags that cost different amounts can come out close, or the cheaper bag can cost more per gallon if it runs out faster.
Include the hidden costs
Hauling heavy bags, storage space, and the time spent refilling count against the bag price, even though they never appear on the receipt. If you buy in bulk to save on hauling, make sure the bulk price holds up per pound once you divide it out. Also keep an eye on residue. A salt that leaves more sludge can mean more frequent brine tank cleaning, and that work has its own cost in time or in a service visit.