Currently, I buy a 40lb bag of finely granulated salt at our local Pinch-a-Penny for $10 a bag. A different Pinch-a-Penny sells coarser salt for $8/bag. My equipment manufacturer (Jandy) says to avoid salt with Yellow Prussiate (I think that's what it's called?), and the Pinch-a-Penny salt doesn't have that ingredient in it (another nearer pool store sells salt with that Yellow stuff in it).

What's the cheapest way to buy large amounts of salt that work properly with a SWCG, and don't have that Yellow Prussiate in it?