$20/375 = .0533 cents per round; cci tactical.
$6/100 = .06 cents per round; cci minimag.
the only time you'll see $6 a box is when you buy in large bulk as mentioned, or gander has one of their $5.99 in store sales. otherwise it is usually 7 cents a round, or more. most gun shops sell it over $8 a box per 100.