You're not wrong-these are usually at least $1 (and often $3 or so) each in stores. I don't believe this is a pricing mistake though. See, you're buying a retail pack of batteries in the store. Here you're buying them bulk-packed. These batteries are SIGNIFICANTLY cheaper in quantity-in the past I've bought 30 CR2032 batteries for $9 or so.
If you're going to buy batteries, let me reccomend the CR2032 batteries, they are by far the most popular memory backup battery in electronics devices, I use them all the time.