I just wanted to bump up the Palmetto State Armory name. I bought some ammo from them, and they email me their deals. They regularly have 5.56 ammo (XM193 or 855) that breaks the $.30/round threshold that I try to seek out, many times with free shipping. I just ordered the 420 round ammo can, 30 cents per round, including the ammo can and free shipping (Federal American Eagle, XM193). I won't post a link, because they constantly change, but it might be worth signing up for their email flyer if you buy a lot of ammo.
I seem to remember a few weeks back they had a deal for 10 5.56 boxes of Federal, along with 10 MagPul magazines for $120 shipped - that's $6 per box of 20 rounds and $6 per MagPul magazine, which was a heck of a deal.