Local homebrew shops are not going to sell you a bag of grain at a loss unless it is old or expired. They small businesses that are in business to make money. Sure they may not make 50%-100% profit off the sack of grain like many of the other items they sell in the store but they are making a profit. Example a shop I used to frequent paid wholesale $32.50 per sack of Briess 2 row they sold for $40. Not a huge mark up but a profit regardless. BTW their price per bag would have been lower if they were able to purchase in larger quantities from their malt supplier.
Edit: wow did not pay attention to the date on this