With free shipping, a 600rd case for $169.50 (28¢ a round) after American Eagle rebate.
https://www.aimsurplus.com/product.aspx?item=AFEDXM855CASE600
Federal American Eagle 5.56x45 (.223) XM855 Ammunition. Features a green tipped, full copper jacketed, 62grn steel core bullet; Brass case and non-corrosive, reloadable, boxer primer. Neatly packaged 150rd box, 4 boxes per case.
https://www.aimsurplus.com/product.aspx?item=AFEDXM855CASE600