Rick Blaine wrote:

....

That's kind of my feeling. NaCl runs around $5 a 40# bag, KCl runs $7 or so. Not
worth worrying about.
....
While it is not a great expense, KCl is about twice the price.
Potassium is heavier so you get fewer molecules per pound.
Specifically, a pound of NaCl has 1.27 times as many molecules
(and softening power) and a pound of KCl.

Around here, the BORG seems cheapest for KCl at $8.29 per 40#.
NaCl is $4.49. If the NaCl bags are also 40#, that would be
2.35 times the price, but I think that the NaCl bags are 50#,
so that would put KCl at 2.94 times the price.