$5/box of 20rds is about the right price for those. You actually have a good price, but not everyone likes the steel-cased ammo for their AR's. I used to run them in mine, but it sticks and causes jams after a while so I don't use steel-cased anymore.
Post it on local board like Armslist, or maybe sell it to a gunshop or pawnshop. You can probably still fetch close to $10/box if you did that.