Today, I went to buy gypsum to apply to my clay soil before the winter rains begin. While I never buy plants at a hardware store or lumber yard, I do buy tools and supplies at such stores.
I went to a nearby hardware store (about 3 miles from my house) and took a 40-pound sack of gypsum to the register. With a senior-citizen discount of 5%, the clerk said that it would be $12+change. I left the sack at the register and drove to a favorite nursery about 12 miles away. There, I bought a 50-pound sack of gypsum for under $9.
That is correct: 25% more gypsum for 25% less money. Both were 92% CaSO4 (calcium sulfate).