We have the same thing at our Grange. The pretty little bags with the cut pic of a chicken on the shelves are 5lbs and about 5-6 dollars, I think. OR, you can get the huge brown sack from the warehouse, no cute pic of a hen on the side and those annoying string closures, fifty pounds for I'm thinking 13 dollars. It's not like the stuff goes bad, so I've got a huge sack sitting in the hay barn.
People with only 1-2 birds don't want to lug around a 50 pound bag. They want a little bag. So they get to pay for the extra handling and extra packaging.
Around here, you go into the feed store and pay nearly as much for a Ziploc of oyster shell that weighs 3 pounds as you'd pay for the 50 pound bag. I don't think it has much to do with brand names, since the Ziploc doesn't have any label at all and one of the store's employees has scooped some oyster shell out and put it into a small package.
I made the mistake of buying the little bag one time, think it was $5.99. But since have bought the 50lb for $8.99 like a 10 months ago. Same thing with cracked corn at the feed store I go to. Its like a 10/15lb bag for $7.50 thats geared towards the backyard feeder people and a 50lb for $11. But anytime you buy volume its going to be a better deal. Convienence= costly- plain and simple.