Seth, this is not a direct answer to your question, but it might help...
If you do not mind the ride, take a drive to Lancaster, PA to visit That FishPlace. I wnet there a couple of years ago and stocked up all all kinds of stuff - heaters, powerheads, salt. Great prices.
Sometimes Petco, yes Petco, has a sale on salt. I picked up a 200 gallon IO pail for $40.00 a few months back. I have not seen it on sale since, however.
As I said, that is not a straight answer to your question, but hopefully it gives you a couple ideas to check out.