LTDAmmo has some for $329.99 + shipping. That comes to roughly $6.90 per box. I love online vendors that have reasonable shipping prices.
SGCUSA has the same for $307.50 but shipping is expensive, even to the West Coast.
I've found PMC to be quality brass-cased ammo. It's a little dirty, but then again with an AR you're shitting where you eat anyway.